These are my personal favorites:
- Moved from Less to Sass.
- Improved grid system.
- Opt-in flexbox support is here.
- Dropped IE8 support and moved to rem and em units.
They are also launching official Bootstrap commercial themes, which I'm sure you'll start seeing absolutely everywhere. They are $99 a pop but quite nice. I imagine the “WordPress versions” of these popping up quite soon, by someone.
The changes in Bootstrap 4 make me want to try it again. A few older features made it a non-starter for me but now — especially with Sass support and the switch from pixels — I'm interested again.