Modification of judetucker's select2-bootstrap-v4.0.0-alpha.6.scss to fix Bootstrap border-radius issue. This uses the Bootstrap border-radius mixin to ensure $enable-rounded is honored.

select2-bootstrap-v4.0.0-alpha.6.scss

//
// Mixins - we need to override the Bootstrap mixins to set border-radius to 0 if $enable-rounded is false.
// --------------------------------------------------


@mixin border-radius($radius: $border-radius) {
    @if $enable-rounded {
        border-radius: $radius;
    }
    @else {
        border-radius: 0;
    }
}

@mixin border-right-radius($radius) {
    @if $enable-rounded {
        border-bottom-right-radius: $radius;
        border-top-right-radius: $radius;
    }
    @else {
        border-radius: 0;
    }
}

@mixin border-left-radius($radius) {
    @if $enable-rounded {
        border-bottom-left-radius: $radius;
        border-top-left-radius: $radius;
    }
    @else {
        border-radius: 0;
    }
}



//
// Variables
// --------------------------------------------------


/* To avoid mixing em and rem units during compilation */
$caret-width: .3rem;

// Variables directly translating Bootstrap variables
// -------------------------

$s2bs-border-radius-base: $border-radius !default;
$s2bs-border-radius-large: $border-radius-lg !default;
$s2bs-border-radius-small: $border-radius-sm !default;
$s2bs-btn-default-bg: $btn-info-bg !default;
$s2bs-btn-default-border: $btn-info-border !default;
$s2bs-btn-default-color: $btn-info-color !default;
$s2bs-caret-width-base: $caret-width !default;
$s2bs-caret-width-large: $caret-width !default;
$s2bs-cursor-disabled: $cursor-disabled !default;
$s2bs-dropdown-header-color: $dropdown-header-color !default;
$s2bs-dropdown-link-active-bg: $dropdown-link-active-bg !default;
$s2bs-dropdown-link-active-color: $dropdown-link-active-color !default;
$s2bs-dropdown-link-disabled-color: $dropdown-link-disabled-color !default;
$s2bs-dropdown-link-hover-bg: $dropdown-link-hover-bg !default;
$s2bs-dropdown-link-hover-color: $dropdown-link-hover-color !default;
$s2bs-font-size-base: $font-size-base !default;
$s2bs-font-size-large: $font-size-lg !default;
$s2bs-font-size-small: $font-size-sm !default;
$s2bs-padding-base-horizontal: $input-padding-x !default;
$s2bs-padding-large-horizontal: $input-padding-x-lg !default;
$s2bs-padding-small-horizontal: $input-padding-x-sm !default;
$s2bs-padding-base-vertical: $input-padding-y !default;
$s2bs-padding-large-vertical: $input-padding-y-lg !default;
$s2bs-padding-small-vertical: $input-padding-y-sm !default;
$s2bs-line-height-base: $input-line-height !default;
$s2bs-line-height-large: $line-height-lg !default;
$s2bs-line-height-small: $line-height-sm !default;
$s2bs-input-bg: $input-bg !default;
$s2bs-input-bg-disabled: $input-bg-disabled !default;
$s2bs-input-color: $input-color !default;
$s2bs-input-color-placeholder: $input-color-placeholder !default;
$s2bs-input-border: $input-border-color !default;
$s2bs-input-border-focus: $input-border-focus !default;
$s2bs-input-border-radius: $border-radius !default;
$s2bs-input-height-base: $input-height !default;
$s2bs-input-height-large: $input-height-lg !default;
$s2bs-input-height-small: $input-height-sm !default;
$s2bs-screen-size-sm: 540px !default;


// Theme-specific variables
// -------------------------

$s2bs-dropdown-arrow-color: $s2bs-input-color-placeholder !default;
$s2bs-dropdown-box-shadow: 0 6px 12px rgba(0,0,0,.175) !default;
$s2bs-dropdown-box-shadow-above: 0px -6px 12px rgba(0,0,0,.175) !default;
$s2bs-clear-selection-color: $s2bs-dropdown-arrow-color !default;
$s2bs-clear-selection-hover-color: $s2bs-btn-default-color !default;
$s2bs-remove-choice-color: $s2bs-btn-default-color !default;
$s2bs-remove-choice-hover-color: $s2bs-btn-default-color !default;
$s2bs-selection-choice-border-radius: $s2bs-border-radius-base !default;
$s2bs-dropdown-header-padding-vertical: $s2bs-padding-base-vertical !default;
$s2bs-dropdown-header-font-size: $s2bs-font-size-small !default;


// Bootstrap defaults not available as pre-defined variables
// These should not be overridden.
// -------------------------

// @see https://github.com/twbs/bootstrap-sass/blob/master/assets/stylesheets/bootstrap/_forms.scss#L126
$s2bs-form-control-default-box-shadow: inset 0 1px 1px rgba(0,0,0,.075) !default;

// @see https://github.com/twbs/bootstrap-sass/blob/master/assets/stylesheets/bootstrap/mixins/_forms.scss#L60
$s2bs-form-control-focus-box-shadow: $s2bs-form-control-default-box-shadow, 0 0 8px rgba($s2bs-input-border-focus, 0.6) !default;

// @see https://github.com/twbs/bootstrap-sass/blob/master/assets/stylesheets/bootstrap/_forms.scss#L127
$s2bs-form-control-transition: border-color ease-in-out .15s, box-shadow ease-in-out .15s !default;




//
// Mixins
// --------------------------------------------------

// @see https://github.com/twbs/bootstrap-sass/blob/master/assets/stylesheets/bootstrap/_forms.scss#L115
@mixin bootstrap-input-defaults {
    @include box-shadow($s2bs-form-control-default-box-shadow);
    background-color: $s2bs-input-bg;
    border: 1px solid $s2bs-input-border;
    color: $s2bs-input-color;
    font-size: $s2bs-font-size-base;
    @include border-radius($s2bs-input-border-radius);
}

// @see http://getbootstrap.com/css/#forms-control-validation
// @see https://github.com/twbs/bootstrap-sass/blob/master/assets/stylesheets/bootstrap/_forms.scss#L388
@mixin validation-state-focus($color) {
    $shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 6px lighten($color, 20%);

    .select2-dropdown,
    .select2-selection {
        border-color: $color;
    }

    .select2-container--focus .select2-selection,
    .select2-container--open .select2-selection {
        @include box-shadow($shadow);
        border-color: darken($color, 10%);
    }

    &.select2-drop-active {
        border-color: darken($color, 10%);

        &.select2-drop.select2-drop-above {
            border-top-color: darken($color, 10%);
        }
    }
}

// dropdown arrow when dropdown is open
@mixin dropdown-arrow {
    .select2-selection--single {
        /**
         * Make the dropdown arrow point up while the dropdown is visible.
         */
        .select2-selection__arrow b {
            border-color: transparent transparent $s2bs-dropdown-arrow-color transparent;
            border-width: 0 $s2bs-caret-width-large $s2bs-caret-width-large $s2bs-caret-width-large;
        }
    }
}





.select2-container--bootstrap {
    display: block;
    /*------------------------------------*\
      #COMMON STYLES
  \*------------------------------------*/
    .select2-selection {
        @include bootstrap-input-defaults;
        outline: 0;

        &.form-control {
            @include border-radius($s2bs-input-border-radius);
        }
    }
    /**
    * Search field in the Select2 dropdown.
    */
    .select2-search--dropdown {
        .select2-search__field {
            @include bootstrap-input-defaults;
        }
    }
    /**
    * No outline for all search fields - in the dropdown
    * and inline in multi Select2s.
    */
    .select2-search__field {
        outline: 0;

        &::-webkit-input-placeholder {
            color: $s2bs-input-color-placeholder;
        }
        /* Firefox 18- */
        &:-moz-placeholder {
            color: $s2bs-input-color-placeholder;
        }
        /**
        * Firefox 19+
        *
        * @see http://stackoverflow.com/questions/24236240/color-for-styled-placeholder-text-is-muted-in-firefox
        */
        &::-moz-placeholder {
            color: $s2bs-input-color-placeholder;
            opacity: 1;
        }

        &:-ms-input-placeholder {
            color: $s2bs-input-color-placeholder;
        }
    }
    /**
    * Adjust Select2's choices hover and selected styles to match
    * Bootstrap 3's default dropdown styles.
    *
    * @see http://getbootstrap.com/components/#dropdowns
    */
    .select2-results__option {
        padding: $s2bs-padding-base-vertical $s2bs-padding-base-horizontal;

        &[role=group] {
            padding: 0;
        }
        /**
        * Disabled results.
        *
        * @see https://select2.github.io/examples.html#disabled-results
        */
        &[aria-disabled=true] {
            color: $s2bs-dropdown-link-disabled-color;
            cursor: $s2bs-cursor-disabled;
        }
        /**
        * Hover state.
        */
        &[aria-selected=true] {
            background-color: $s2bs-dropdown-link-hover-bg;
            color: $s2bs-dropdown-link-hover-color;
        }
        /**
        * Selected state.
        */
        &--highlighted[aria-selected] {
            background-color: $s2bs-dropdown-link-active-bg;
            color: $s2bs-dropdown-link-active-color;
        }

        .select2-results__option {
            padding: $s2bs-padding-base-vertical $s2bs-padding-base-horizontal;

            .select2-results__group {
                padding-left: 0;
            }

            .select2-results__option {
                margin-left: -$s2bs-padding-base-horizontal;
                padding-left: $s2bs-padding-base-horizontal*2;

                .select2-results__option {
                    margin-left: -$s2bs-padding-base-horizontal*2;
                    padding-left: $s2bs-padding-base-horizontal*3;

                    .select2-results__option {
                        margin-left: -$s2bs-padding-base-horizontal*3;
                        padding-left: $s2bs-padding-base-horizontal*4;

                        .select2-results__option {
                            margin-left: -$s2bs-padding-base-horizontal*4;
                            padding-left: $s2bs-padding-base-horizontal*5;

                            .select2-results__option {
                                margin-left: -$s2bs-padding-base-horizontal*5;
                                padding-left: $s2bs-padding-base-horizontal*6;
                            }
                        }
                    }
                }
            }
        }
    }

    .select2-results__group {
        color: $s2bs-dropdown-header-color;
        display: block;
        padding: $s2bs-dropdown-header-padding-vertical $s2bs-padding-base-horizontal;
        font-size: $s2bs-dropdown-header-font-size;
        line-height: $s2bs-line-height-base;
        white-space: nowrap;
    }

    &.select2-container--focus,
    &.select2-container--open {
        .select2-selection {
            @include box-shadow($s2bs-form-control-focus-box-shadow);
            @include transition($s2bs-form-control-transition);
            border-color: $s2bs-input-border-focus;
        }
    }

    &.select2-container--open {
        /**
         * Make the dropdown arrow point up while the dropdown is visible.
         */
        .select2-selection .select2-selection__arrow b {
            border-color: transparent transparent $s2bs-dropdown-arrow-color transparent;
            border-width: 0 $s2bs-caret-width-base $s2bs-caret-width-base $s2bs-caret-width-base;
        }
        /**
         * Handle border radii of the container when the dropdown is showing.
         */
        &.select2-container--below {
            .select2-selection {
                @include border-bottom-radius(0);
                border-bottom-color: transparent;
            }
        }

        &.select2-container--above {
            .select2-selection {
                @include border-top-radius(0);
                border-top-color: transparent;
            }
        }
    }
    /**
    * Clear the selection.
    */
    .select2-selection__clear {
        color: $s2bs-clear-selection-color;
        cursor: pointer;
        float: right;
        font-weight: bold;
        margin-right: 10px;

        &:hover {
            color: $s2bs-clear-selection-hover-color;
        }
    }
    /**
    * Address disabled Select2 styles.
    *
    * @see https://select2.github.io/examples.html#disabled
    * @see http://getbootstrap.com/css/#forms-control-disabled
    */
    &.select2-container--disabled {
        .select2-selection {
            border-color: $s2bs-input-border;
            @include box-shadow(none);
        }

        .select2-selection,
        .select2-search__field {
            cursor: $s2bs-cursor-disabled;
        }

        .select2-selection,
        .select2-selection--multiple .select2-selection__choice {
            background-color: $s2bs-input-bg-disabled;
        }

        .select2-selection__clear,
        .select2-selection--multiple .select2-selection__choice__remove {
            display: none;
        }
    }
    /*------------------------------------*\
      #DROPDOWN
    \*------------------------------------*/
    /**
    * Dropdown border color and box-shadow.
    */
    .select2-dropdown {
        @include box-shadow($s2bs-dropdown-box-shadow);
        border-color: $s2bs-input-border-focus;
        overflow-x: hidden;
        margin-top: -1px;
        /*here1!*/
        @include border-radius($s2bs-input-border-radius);
        /*here1!*/
        &--above {
            @include box-shadow($s2bs-dropdown-box-shadow-above);
            margin-top: 1px;
        }
    }
    /**
    * Limit the dropdown height.
    */
    .select2-results > .select2-results__options {
        max-height: 200px;
        overflow-y: auto;
    }
    /*------------------------------------*\
      #SINGLE SELECT2
    \*------------------------------------*/
    .select2-selection--single {
        height: $s2bs-input-height-base;
        line-height: $s2bs-line-height-base;
        padding: $s2bs-padding-base-vertical ($s2bs-padding-base-horizontal + $s2bs-caret-width-base*3) $s2bs-padding-base-vertical $s2bs-padding-base-horizontal;
        /**
        * Adjust the single Select2's dropdown arrow button appearance.
        */
        .select2-selection__arrow {
            position: absolute;
            bottom: 0;
            right: $s2bs-padding-base-horizontal;
            top: 0;
            width: $s2bs-caret-width-base;

            b {
                border-color: $s2bs-dropdown-arrow-color transparent transparent transparent;
                border-style: solid;
                border-width: $s2bs-caret-width-base $s2bs-caret-width-base 0 $s2bs-caret-width-base;
                height: 0;
                left: 0;
                margin-left: -$s2bs-caret-width-base;
                margin-top: -$s2bs-caret-width-base/2;
                position: absolute;
                top: 50%;
                width: 0;
            }
        }

        .select2-selection__rendered {
            color: $s2bs-input-color;
            padding: 0;
        }

        .select2-selection__placeholder {
            color: $s2bs-input-color-placeholder;
        }
    }
    /*------------------------------------*\
    #MULTIPLE SELECT2
    \*------------------------------------*/
    .select2-selection--multiple {
        min-height: $s2bs-input-height-base;
        padding: 0;
        height: auto;

        .select2-selection__rendered {
            box-sizing: border-box;
            display: block;
            line-height: $s2bs-line-height-base;
            list-style: none;
            margin: 0;
            overflow: hidden;
            padding: 0;
            width: 100%;
            text-overflow: ellipsis;
            white-space: nowrap;
        }

        .select2-selection__placeholder {
            color: $s2bs-input-color-placeholder;
            float: left;
            margin-top: 5px;
        }
        /**
         * Make Multi Select2's choices match Bootstrap 3's default button styles.
         */
        .select2-selection__choice {
            color: $s2bs-btn-default-color;
            background: $s2bs-btn-default-bg;
            border: 1px solid $s2bs-btn-default-border;
            cursor: default;
            float: left;
            margin: -($s2bs-padding-base-vertical - 1) 0 0 $s2bs-padding-base-horizontal/2;
            padding: 0 $s2bs-padding-base-vertical;
            @include border-radius($s2bs-selection-choice-border-radius);
        }
        /**
         * Minus 2px borders.
         */
        .select2-search--inline {
            .select2-search__field {
                background: transparent;
                padding: 0 $s2bs-padding-base-horizontal;
                height: $s2bs-input-height-base;
                line-height: $s2bs-line-height-base;
                margin-top: 0;
                min-width: 5em;
            }
        }

        .select2-selection__choice__remove {
            color: $s2bs-remove-choice-color;
            cursor: pointer;
            display: inline-block;
            font-weight: bold;
            margin-right: $s2bs-padding-base-vertical / 2;

            &:hover {
                color: $s2bs-remove-choice-hover-color;
            }
        }
        /**
         * Clear the selection.
         */
        .select2-selection__clear {
            margin-top: $s2bs-padding-base-vertical;
        }
    }
    /**
    * Address Bootstrap control sizing classes
    *
    * 1. Reset Bootstrap defaults.
    * 2. Adjust the dropdown arrow button icon position.
    *
    * @see http://getbootstrap.com/css/#forms-control-sizes
    */
    /* 1 */
    .select2-selection--single.input-sm,
    .input-group-sm & .select2-selection--single,
    .form-group-sm & .select2-selection--single {
        @include border-radius($s2bs-border-radius-small);
        font-size: $s2bs-font-size-small;
        height: $s2bs-input-height-small;
        line-height: $s2bs-line-height-small;
        padding: $s2bs-padding-small-vertical $s2bs-padding-small-horizontal + $s2bs-caret-width-base*3 $s2bs-padding-small-vertical $s2bs-padding-small-horizontal;
        /* 2 */
        .select2-selection__arrow b {
            margin-left: -$s2bs-padding-small-vertical;
        }
    }

    .select2-selection--multiple.input-sm,
    .input-group-sm & .select2-selection--multiple,
    .form-group-sm & .select2-selection--multiple {
        min-height: $s2bs-input-height-small;
        @include border-radius($s2bs-border-radius-small);

        .select2-selection__choice {
            font-size: $s2bs-font-size-small;
            line-height: $s2bs-line-height-small;
            margin: ($s2bs-padding-small-vertical) 0 0 $s2bs-padding-small-horizontal/2;
            padding: 0 $s2bs-padding-small-vertical;
        }

        .select2-search--inline .select2-search__field {
            padding: 0 $s2bs-padding-small-horizontal;
            font-size: $s2bs-font-size-small;
            height: $s2bs-input-height-small - 2;
            line-height: $s2bs-line-height-small;
        }

        .select2-selection__clear {
            margin-top: $s2bs-padding-small-vertical;
        }
    }

    .select2-selection--single.input-lg,
    .input-group-lg & .select2-selection--single,
    .form-group-lg & .select2-selection--single {
        @include border-radius($s2bs-border-radius-large);
        font-size: $s2bs-font-size-large;
        height: $s2bs-input-height-large;
        line-height: $s2bs-line-height-large;
        padding: $s2bs-padding-large-vertical $s2bs-padding-large-horizontal + $s2bs-caret-width-large*3 $s2bs-padding-large-vertical $s2bs-padding-large-horizontal;
        /* 1 */
        .select2-selection__arrow {
            width: $s2bs-caret-width-large;

            b {
                border-width: $s2bs-caret-width-large $s2bs-caret-width-large 0 $s2bs-caret-width-large;
                margin-left: -$s2bs-caret-width-large;
                margin-left: -$s2bs-padding-large-vertical;
                margin-top: -$s2bs-caret-width-large/2;
            }
        }
    }

    .select2-selection--multiple.input-lg,
    .input-group-lg & .select2-selection--multiple,
    .form-group-lg & .select2-selection--multiple {
        min-height: $s2bs-input-height-large;
        @include border-radius($s2bs-border-radius-large);

        .select2-selection__choice {
            font-size: $s2bs-font-size-large;
            line-height: $s2bs-line-height-large;
            @include border-radius($s2bs-selection-choice-border-radius);
            margin: ($s2bs-padding-large-vertical - 1) 0 0 $s2bs-padding-large-horizontal/2;
            padding: 0 $s2bs-padding-large-vertical;
        }

        .select2-search--inline .select2-search__field {
            padding: 0 $s2bs-padding-large-horizontal;
            font-size: $s2bs-font-size-large;
            height: $s2bs-input-height-large - 2;
            line-height: $s2bs-line-height-large;
        }

        .select2-selection__clear {
            margin-top: $s2bs-padding-large-vertical;
        }
    }

    .select2-selection.input-lg.select2-container--open {
        @include dropdown-arrow;
    }

    .input-group-lg & .select2-selection {
        &.select2-container--open {
            @include dropdown-arrow;
        }
    }
    /*------------------------------------*\
    #RTL SUPPORT
    \*------------------------------------*/
    &[dir="rtl"] {
        /**
        * Single Select2
        *
        * 1. Makes sure that .select2-selection__placeholder is positioned
        *    correctly.
        */
        .select2-selection--single {
            padding-left: $s2bs-padding-base-horizontal + $s2bs-caret-width-base*3;
            padding-right: $s2bs-padding-base-horizontal;

            .select2-selection__rendered {
                padding-right: 0;
                padding-left: 0;
                text-align: right; /* 1 */
            }

            .select2-selection__clear {
                float: left;
            }

            .select2-selection__arrow {
                left: $s2bs-padding-base-horizontal;
                right: auto;

                b {
                    margin-left: 0;
                }
            }
        }
        /**
         * Multiple Select2
         */
        .select2-selection--multiple {
            .select2-selection__choice,
            .select2-selection__placeholder,
            .select2-search--inline {
                float: right;
            }

            .select2-selection__choice {
                margin-left: 0;
                margin-right: $s2bs-padding-base-horizontal/2;
            }

            .select2-selection__choice__remove {
                margin-left: 2px;
                margin-right: auto;
            }
        }
    }
}





/*------------------------------------*\
  #ADDITIONAL GOODIES
\*------------------------------------*/

/**
 * Address Bootstrap's validation states
 *
 * If a Select2 widget parent has one of Bootstrap's validation state modifier
 * classes, adjust Select2's border colors and focus states accordingly.
 * You may apply said classes to the Select2 dropdown (body > .select2-container)
 * via JavaScript match Bootstraps' to make its styles match.
 *
 * @see http://getbootstrap.com/css/#forms-control-validation
 */

.has-warning {
    @include validation-state-focus($state-warning-text);
}

.has-error {
    @include validation-state-focus($state-danger-text);
}

.has-success {
    @include validation-state-focus($state-success-text);
}

/**
 * Select2 widgets in Bootstrap Input Groups
 *
 * @see http://getbootstrap.com/components/#input-groups
 * @see https://github.com/twbs/bootstrap/blob/master/less/input-groups.less
 */

/**
 * Reset rounded corners
 */

.input-group > .select2-hidden-accessible {
    &:first-child + .select2-container--bootstrap > .selection > .select2-selection,
    &:first-child + .select2-container--bootstrap > .selection > .select2-selection.form-control {
        @include border-right-radius(0);
    }

    &:not(:first-child) + .select2-container--bootstrap:not(:last-child) > .selection > .select2-selection,
    &:not(:first-child) + .select2-container--bootstrap:not(:last-child) > .selection > .select2-selection.form-control {
        @include border-radius(0);
    }

    &:not(:first-child):not(:last-child) + .select2-container--bootstrap:last-child > .selection > .select2-selection,
    &:not(:first-child):not(:last-child) + .select2-container--bootstrap:last-child > .selection > .select2-selection.form-control {
        @include border-left-radius(0);
    }
}

.input-group > .select2-container--bootstrap {
    display: table;
    table-layout: fixed;
    position: relative;
    z-index: 2;
    width: 100%;
    margin-bottom: 0;

    > .selection > .select2-selection.form-control {
        float: none;
    }
    /**
    * Adjust z-index like Bootstrap does to show the focus-box-shadow
    * above appended buttons in .input-group and .form-group.
    */
    &.select2-container--open, /* .form-group */
    &.select2-container--focus /* .input-group */ {
        z-index: 3;
    }
    /**
    * Adjust alignment of Bootstrap buttons in Bootstrap Input Groups to address
    * Multi Select2's height which - depending on how many elements have been selected -
    * may grow taller than its initial size.
    *
    * @see http://getbootstrap.com/components/#input-groups
    */
    &,
    .input-group-btn,
    .input-group-btn .btn {
        vertical-align: top;
    }
}

/**
 * Temporary fix for https://github.com/select2/select2-bootstrap-theme/issues/9
 *
 * Provides `!important` for certain properties of the class applied to the
 * original `<select>` element to hide it.
 *
 * @see https://github.com/select2/select2/pull/3301
 * @see https://github.com/fk/select2/commit/31830c7b32cb3d8e1b12d5b434dee40a6e753ada
 */

.form-control.select2-hidden-accessible {
    position: absolute !important;
    width: 1px !important;
}

/**
 * Display override for inline forms
 */

.form-inline .select2-container--bootstrap {
    @media (min-width: $s2bs-screen-size-sm) {
        display: inline-block;
    }
}

Apologies, I had no notification you had commented.

I've made a few changes to this, after realising it needs to override the border-radius mixins from Bootstrap, to explicitly set border-radius to 0 if $enable-rounded == false.

I've compiled the css, but this is based on $enable-rounded being false (so no border-radius). If you want rounded corners, either compile the scss yourself or modify thr border-radius properties to match what you want.

Anyway, the css file is here:

https://gist.github.com/DaleMckeown/ce5a8d32bd0c1d44462090eb12c554cc