Column breaks with Bootstrap Grid System

Breaking columns to a new line in flexbox requires a small hack: add an element with width: 100% wherever you want to wrap your columns to a new line. Normally this is accomplished with multiple .rows, but not every implementation method can account for this.

<div class="container">
  <div class="row">
    <div class="col-6 col-sm-3">.col-6 .col-sm-3</div>
    <div class="col-6 col-sm-3">.col-6 .col-sm-3</div>

    <!-- Force next columns to break to new line -->
    <div class="w-100"></div>

    <div class="col-6 col-sm-3">.col-6 .col-sm-3</div>
    <div class="col-6 col-sm-3">.col-6 .col-sm-3</div>
  </div>
</div>
<div class="container">
  <div class="row">
    <div class="col-6 col-sm-4">.col-6 .col-sm-4</div>
    <div class="col-6 col-sm-4">.col-6 .col-sm-4</div>

    <!-- Force next columns to break to new line at md breakpoint and up -->
    <div class="w-100 d-none d-md-block"></div>

    <div class="col-6 col-sm-4">.col-6 .col-sm-4</div>
    <div class="col-6 col-sm-4">.col-6 .col-sm-4</div>
  </div>
</div>

References
https://getbootstrap.com/docs/4.3/layout/grid/#column-breaks