Wells are dropped from Bootstrap with the version 4.
You can use Cards instead of Wells.
Here is a quick example for how to use new Cards feature:
<div class="card card-inverse" style="background-color: #333; border-color: #333;">
<div class="card-block">
<h3 class="card-title">Card Title</h3>
<p class="card-text">This text will be written on a grey background inside a Card.</p>
<a href="#" class="btn btn-primary">This is a Button</a>
</div>
</div>