Cards

What you notice first in Bootstrap 4 is that .panel and .well have been replaced by the .card. This makes a lot of sense as "cards" are one of the more well known trends in responsive design. The Bootstrap 4 Cards can be singular, grouped together (as shown) or equal height. There are many different layout and content options.

Consise

Cards are a good way to display content composed of different types of objects. They’re well-suited for presenting similar objects whose size or supported actions vary, like headings and photos with captions.

Grouped

Use card groups to render equal height cards without gutters between the cards. Use .card-deck for cards that aren’t attached to each another.

Same Height

A frequent problem occurs when you have multiple items in a row with content of varying height. Thanks to flexbox (and display:table) we now have equal height cards!

Fonts got larger

Wow.

Text is larger in Bootstrap 4. The default font size is now 16px. There are also new "Display headings" that really stand out. The new `em` based sizing is cool too.

Navbar

The new Bootstrap 4 .navbar code is lighter. You can make it responsive using the Collapse component and .navbar-toggleable-* classes that enable you to control the breakpoint.

RIP icons, affix, etc..

Some components have been removed from Bootstrap 4. The Glyphicons icon fonts are no longer included, but it's easy to utilize another font like FontAwesome, Octicons or Ionicons. The Affix jQuery plugin has also been dropped. The new recommendation is to use HTML5 position: sticky or a sticky polyfill instead.

text-capitalize Capitalize each word
text-uppercase Uppercase text
text-lowercase There is lowercase too
But where is text-truncate with text-overflow:ellipsis?

Migration tips

text-center = text-xs-center
text-right = text-xs-right
pull-right = pull-xs-right
img-responsive = img-fluid
list-inline > li = list-inline-item
add nav-item to nav navbar > li
add nav-link to nav navbar > li > a
the active class is added to nav-item for scrollspy
carousel-inner > item = carousel-item
table-condensed = table-sm
center-block = m-x-auto

More on What's New in Bootstrap 4
Upgrade to Bootstrap 4

Handy utilities

Several simple, but useful utility classes have been added like text, margin and padding helpers. There are also contextual colors and responsive utility classes to hide content on specific breakpoints. There's even a new fluid Jumbotron!

What's New in Bootstrap 4?

A free, one-page theme to highlight the new features

Based on 4.0-alpha.4

Cards

Consise

Grouped

Same Height

Flexbox

Easier

Sass-y

It's Modern

Now there are 5 Breakpoints

XS

<544 px

SM

≥544 px

MD

≥768 px

LG

≥992 ox

XL