Bootstrap 4 is on the way and was announced this past Wednesday. One major change to Bootstrap is that it is dropping support for Internet Explorer 8, which means less hacking for backwards compatibility. Here’s an excerpt from the Bootstrap Blog:
Dropped IE8 support and moved to rem and em units. Dropping support for IE8 means we can take advantage of the best parts of CSS without being held back with CSS hacks or fallbacks. Pixels have been swapped for rems and ems where appropriate to make responsive typography and component sizing even easier. If you need IE8 support, keep using Bootstrap 3.
We’re playing around with it and looking forward to seeing how developers use v4 for Website development, backend UI, and, mobile applications.