Original Link: http://blog.getbootstrap.com/2015/08/19/bootstrap-4-alpha/

Major changes in Version 4. Some highlights:

  • Moved to Sass from LESS
  • Improved version of box-sizing as part of their new reset.
  • Opt-in (single variable toggle) to use flexbox on the grids/components.
  • Type set mostly in rems
  • All JavaScript in ES6

And another big move for them: monitization through purchaseable themes designed by Bootstrap team themselves.