Panels, thumbnails, and wells
Dropped entirely for the new card component.

Panels
.panel to .card, now built with flexbox.
.panel-default removed and no replacement.
.panel-group removed and no replacement. .card-group is not a replacement, it is different.
.panel-heading to .card-header
.panel-title to .card-title. Depending on the desired look, you may also want to use heading elements or classes (e.g. <h3>, .h3) or bold elements or classes (e.g. <strong>, <b>, .font-weight-bold). Note that .card-title, while similarly named, produces a different look than .panel-title.
.panel-body to .card-block
.panel-footer to .card-footer
.panel-primary to .card-primary and .card-inverse (or use .bg-primary on .card-header)
.panel-success to .card-success and .card-inverse (or use .bg-success on .card-header)
.panel-info to .card-info and .card-inverse (or use .bg-info on .card-header)
.panel-warning to .card-warning and .card-inverse (or use .bg-warning on .card-header)
.panel-danger to .card-danger and .card-inverse (or use .bg-danger on .card-header)

這邊
https://bootstrap.hexschool.com/docs/4.0/migration/#panels-thumbnails-and-wells

取代的是card
https://bootstrap.hexschool.com/docs/4.0/components/card/