Espaciado Bootstrap con h-100

<div class="container">
  <div class="row">
    <div class="col-md-4 col-sm-6 mb-4">
      <div class="card h-100" style="background-color:#ff0000;">
        Test card content. 
      </div>
    </div>
    <div class="col-md-4 col-sm-6 mb-4">
      <div class="card h-100" style="background-color:#00ff00;">
        Test card content. Test card content. Test card content. Test card content. 
      </div>
    </div>
    <div class="col-md-4 col-sm-6 mb-4">
      <div class="card h-100" style="background-color:#0000ff;">
        Test card content. Test card content. Test card content. Test card content. Test card content. Test card content. Test card content. Test card content. 
      </div>
    </div>
    <div class="col-md-4 col-sm-6 mb-4">
      <div class="card h-100" style="background-color:#0f0f0f;">
        Test card content. 
      </div>
    </div>
  </div>
</div>

La solución vista en Stackoverflow.