Bootstrap is the most popular HTML, CSS, and JS framework in the world for building responsive, mobile-first projects on the web. Bootstrap available on MIT license, that makes its most lovable framework of front end developer. Bootstrap angular version also available on market, so you can use bootstrap component (model, tool tip) as angular directive not need to include explicitly on your angular project.
Bootstrap announced bootstrap 4 with alpha release on 19th august 2015. There are a lot of new features coming on way due to its alpha release. I have listed important features of bootstrap 4.
1. Dropped IE8 support
2. Less to Sass
3. Improved Media Queries
5. Improved Grid System
6. Removed panels, wells and thumbnails
8. Display Headings
9. Support jQuery 2.0
10. Opt-in flex-box support.
11. Improved auto-placement of tooltips and popovers
12. Improved documentation