Bootstrap 4 Callouts pulled from https://v4-alpha.getbootstrap.com/assets/css/docs.min.css

_callout.scss

.callout {
  padding: 1.25rem;
  margin-top: 1.25rem;
  margin-bottom: 1.25rem;
  border: $card-border-width solid $card-border-color;
  border-left-width: .25rem;
  @include border-radius($card-border-radius);

  h2,h3,h4,h5 {
    margin-top: 0;
    margin-bottom: $spacer;
  }

  p:last-child {
    margin-bottom: 0;
  }

  code {
    border-radius: .25rem;
  }
}

@each $color, $value in $theme-colors {
  .callout-#{$color} {
    border-left-color: $value;

    h2,h3,h4,h5 {
      color: $value;
    }
  }
}

Depending on your setup, you might need to add @import 'bootstrap/variables'; or similar at the top. I did.