Unreal Engine 4 was released on March 19th 2014. So that means that there's been roughly 3 months between right now and the release of Unreal 4.
While everybody knew Unreal 4 was in development for a while, nobody knew when exactly it was going to release. And from the screens we have of KF2 so far, it seems as though it's been in development for some time. The PC Gamer article correlates this by stating
"Since shipping World War II FPS Red Orchestra in 2011, Tripwire has dedicated itself to the sticky art of digital dismemberment for the sequel to [Killing Floor]"
So, this suggests that development of the new gore engine for Killing Floor 2 (if not the entire game itself) started in 2011, a full 3 years before Unreal 4 was released.
What sense does it make to delay working on a sequel to a popular game by 3 years just to make it look slightly prettier? And if they DID decide to stop working on KF on Unreal 3 and switch to 4, we wouldn't have our hands on a playable version for years. Not worth it, in my opinion.