Modified Grid System
As B4 dropped ie8 support. Pixels have been swapped for rems and ems. now, A container now has max-width set in rems. Due to spacing accuracy in rems grid system will be more mobile devices friendly.
Less to Sass
Bootstrap 4 moved to Sass from Less. Mainly Two main reason, faster compilation and huge community base of Sass.
Completely Redesigned Navbar
Dropped Support for IE8
The Bootstrap team has removed IE8 from the list of supported browsers. One of the main reasons was the transition from pixels to rem/em, which are only partially supported by IE8.
Moved to rem and em units instead of pixels
At Release of (the alpha) version 4 of Bootstrap pixels have been swapped for rem and em where appropriate to make responsive typography and component sizing even easier.
Dropped the Glyphicons font
No explanation given why these were dropped. Based on the Bootstrap 3 section about the Affix I already had the impression support for this plugin was a bit half-hearted.
From Normalize.css to Reboot.css
CSS reset Bootstrap 4 forked Normalize.css and after adding some own style created reboot.css
Improved Auto-Placement of Tooltips and Popovers
Tooltips and Popovers now rely on the 3rd party library Tether for positioning. You must include tether.min.js before bootstrap.js in order for popovers and tooltips to work!
New Class ‘Inverse’ for Tables
Flexbox support Bootstrap. Flexbox provides simpler and more flexible layout
New Cards Option
In the future, the three very similar components wells, thumbnails and panels will be replaced by a component called Cards. This is a good step in reducing the number of components and structuring the framework more clearly.