With the release of Bootstrap 4, a few components have been dropped and a couple of new ones have been added. The most significant change is the new Cards component. Let's start by breaking down this new option.
With the release of the Cards component, the Panels, Thumbnails, and Wells components have been removed from Bootstrap 4. Cards combines the best of these elements into one and even adds some new functionality that is really useful. If you are migrating from a Bootstrap 3 project, you'll need to update any Panels, Thumbnails, or Wells to use the Cards component instead. Since the markup is a bit different, I would recommend just removing the old components altogether, and then recoding them using the same content as Cards.