What is the '.well' equivalent class in Bootstrap 4

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>

Update 2018...

card has replaced the well.

Bootstrap 4

<div class="card card-body bg-light">
     Well
</div>

or, as two DIVs...

<div class="card bg-light">
    <div class="card-body">
        ...
    </div>
</div>

(Note: in Bootstrap 4 Alpha, these were known as card-block instead of card-body and bg-faded instead of bg-light)

http://codeply.com/go/rW2d0qxx08