Bootstrap 4 Spacing for Bootstrap 3

spacing.scss

$grid-breakpoints: (
        xs: 0,
        sm: 576px,
        md: 768px,
        lg: 992px,
        xl: 1200px
) !default;

$spacer: 20px !default;
$spacers: (
        0: 0,
        1: ($spacer * .25),
        2: ($spacer * .5),
        3: $spacer,
        4: ($spacer * 1.5),
        5: ($spacer * 3)
) !default;


@function breakpoint-min($name, $breakpoints: $grid-breakpoints) {
  $min: map-get($breakpoints, $name);
  @return if($min != 0, $min, null);
}

@function breakpoint-infix($name, $breakpoints: $grid-breakpoints) {
  @return if(breakpoint-min($name, $breakpoints) == null, "", "-#{$name}");
}

@mixin media-breakpoint-up($name, $breakpoints: $grid-breakpoints) {
  $min: breakpoint-min($name, $breakpoints);
  @if $min {
    @media (min-width: $min) {
      @content;
    }
  } @else {
    @content;
  }
}

@each $breakpoint in map-keys($grid-breakpoints) {
  @include media-breakpoint-up($breakpoint) {
    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);

    @each $prop, $abbrev in (margin: m, padding: p) {
      @each $size, $length in $spacers {

        .#{$abbrev}#{$infix}-#{$size} { #{$prop}: $length !important; }
        .#{$abbrev}t#{$infix}-#{$size},
        .#{$abbrev}y#{$infix}-#{$size} {
          #{$prop}-top: $length !important;
        }
        .#{$abbrev}r#{$infix}-#{$size},
        .#{$abbrev}x#{$infix}-#{$size} {
          #{$prop}-right: $length !important;
        }
        .#{$abbrev}b#{$infix}-#{$size},
        .#{$abbrev}y#{$infix}-#{$size} {
          #{$prop}-bottom: $length !important;
        }
        .#{$abbrev}l#{$infix}-#{$size},
        .#{$abbrev}x#{$infix}-#{$size} {
          #{$prop}-left: $length !important;
        }
      }
    }

    // Some special margin utils
    .m#{$infix}-auto { margin: auto !important; }
    .mt#{$infix}-auto,
    .my#{$infix}-auto {
      margin-top: auto !important;
    }
    .mr#{$infix}-auto,
    .mx#{$infix}-auto {
      margin-right: auto !important;
    }
    .mb#{$infix}-auto,
    .my#{$infix}-auto {
      margin-bottom: auto !important;
    }
    .ml#{$infix}-auto,
    .mx#{$infix}-auto {
      margin-left: auto !important;
    }
  }
}