4GB is NOT enough for 4K in NEW games what are you talking about??? "Next-Gen" multiplatform games need about 3GB if not more just to run maxed out at 1080p60fps.
If you're buying a new GPU right now and want to max out all incoming games in the near future while keeping a 60fps frame rate, you will need 4GB if not more since PS4 and X1 have about 5GB of memory dedicated to games (expect rushed ports that don't make good use of your whole PC and just fill up your whole VRAM pool)
To OP, 6GB will make your PC very future proof. However, not sure how much that Titan costs, but you would probably be better off just getting a GTX 980 4GB. That will run Witcher 3, Batman Arkham Knight, Assassin's Creed Unity, etc. and all upcoming games BEAUTIFULLY (in 1080p and probably 1440p 60fps that is).