Suzuki@1473780084@1473780163 :
Save money and buy a GTX 980 Ti, also don't have to go reference card so it won't be as loud.
^ THIS ^
A good 980ti with a decent overclock will run like a Titan X, and cost MUCH less. And the new AMD Fury would be right behind it at 4k, again for less than the 980ti.
Still, for 4k, there's not really any good single-GPU that can do 4k at high FPS just yet. The Fury and 980ti would be my two picks for 4k, because you can do a dual-GPU setup for around the same cost as a single TitanX.
Quick Refs:
https://www.youtube.com/watch?v=B2-gCXscH04
https://www.youtube.com/watch?v=miMaW894kjw