Alright, I looked at some reviews for top cards. The top two, I forget which ones said you only need to run 1 of them in your PC to play 4k games at good settings.
I've never read any such review this generation but then it depends greatly on the types of games you play, the level of FPS you require (30 fps avg, 60 fps avg, 60 fps minimums?), your sensitivity to micro-stuttering and what you consider "good" settings. Some people might find gaming 4K on medium @ 30 fps good and others want 4K at very high quality with 60 fps.
Something to keep in mind is games will only get more demanding. If you think you can just purchase 980Ti SLI or Fury X CF today and be set for 4-5 years, then 4K is not for you. 4K like 1440P/1600p of the past will require constant upgrading to flagship cards because. The highest resolution PC gaming has always demanded having the most cutting edge GPUs for never ending demands of next gen PC games or the never ending barrage of poorly optimized console-to-PC ports.
As Silverforce11 alluded to earlier, the level of GPU horsepower required depends widely on the game:
http://www.techspot.com/review/1033-gtx-980-ti-sli-r9-fury-x-crossfire/
IMO, if a game isn't running smooth, the resolution is secondary and if you can't max out most settings, then resolution cannot hide lower quality shaders, textures, shadows, god rays, etc. Therefore, if you can't play 4K at High-Very High @ 60 fps, then I'd much rather pick a 144Hz 1440P or even a 60-100Hz 1440P monitor instead and upgrade to 4K when it's more affordable.