Ubuntu 16.04 LTS Continues To Focus On The Linux 4.4 Kernel
Linux 4.4 is their target for the "Xenial Xerus" since Ubuntu 16.04 is to be a Long-Term Support release and the upstream 4.4 kernel is also being maintained as a long-term release too. Additionally, Linux 4.5 would come too close to the April debut of Ubuntu 16.04 that the developers wouldn't feel comfortable, particularly for an LTS release.
Linux 4.4 has many new features though there will likely be a lot of backporting that happens for the Ubuntu 16.04 kernel... Ubuntu tends to backport a lot of Intel DRM graphics driver code from upstream into their shipped kernels. For Linux 4.4 / Ubuntu 16.04, this will likely mean backporting newer code for Kabylake support so that it hits the Ubuntu release plus who knows whatever other additions from Intel. Ubuntu's kernel team doesn't tend to backport any significant (generally any?) Radeon/Nouveau kernel driver code to their shipping kernel. Sadly missing out then from the Ubuntu 16.04 kernel would be the AMDGPU PowerPlay/re-clocking support and more. Using Linux 4.4 means no KDBUS for Ubuntu 16.04 as well.
Confirmation of the Ubuntu kernel team's re-affirmed target of Linux 4.4 for Ubuntu 16.04 can be found via the kernel team's newsletter.