<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.