As a reminder, Canonical has scheduled the release of Ubuntu 16.04 Alpha 1 on December 31. Until than, they are working at implementing a new default kernel stack.
The final plan is to use Kernel 4.4 as default on Ubuntu 16.04, but the Ubuntu developers have to go in this direction with baby steps, adopt new kernel versions one by one and fix what’s not working properly.
Recently, one of the Ubuntu devs has announced that kernel 4.3 has been added to the master-next branch of Ubuntu 16.04, but kernel 4.4 rc3 is also tracked in parallel, in the unstable repository.
If you want to give Canonical a hand, you should download and test the Ubuntu 16.04 Daily Images and report bugs.