We are currently testing our board support files with Vivado 2014.4, the very first success with converted board part files was within an hour after we installed 2014.4 - a created from scratch design using the ported board part files did work when booting from SPI flash on one of our new SoM's.
I will not comment on 2014.4 new features before the public release, but I can say that the changes compared to 2014.3 are more than I expected. Those changes (those I am already aware of) are mostly in the board part flow support and not so relevant from user prospective, except that we can now really provide the board files without too much "workarounds" and probably less issues when migrating to next versions of Vivado. It also confirms that the decision not to release board support for 2014.3 was the right one (we did deliver 2014.3 only to a few customers who directly requested it).