For those worrying about gaming performance with 4K:
I'm running an older i5-2500K clocked at 3.3Ghz with 16GB ram. I have a single GTX 970 card. I was a bit worried that it would struggle (particularly the ageing CPU), but it runs really well.
Most games I've tried at full resolution have been fine to good. You can lower the AA to 2x to give the game more headroom, and it looks fantastic in most stuff. I have most games on high/ultra settings. Sometimes I turn down shadows slightly to medium/high.
World of Warcraft runs mostly at 60fps, dipping to 40fps in certain areas.
Skyrim runs similarly to World of Warcraft, 60fps to 40fps. When it dips there is no discernable lag or feeling of sluggishness.
Alien Isolation runs smoothly around 50fps.
Divinity: Original Sin looks stunning, I haven't measured but it must be around 60fps on max settings.
I haven't yet tried any twitch shooters like CS:GO but given the above performance, I suspect it'll run at max settings at 50-60fps.
I would like to re-iterate that 4K resolution is absolutely mind blowing. The picture detail and quality of this screen is so gorgeous you won't believe how you lived with 1080p.
I haven't detected any screen tearing or unusual behaviour in any games. Colours are quite cool (i.e. they don't jump out / feel saturated) but they make for a beautiful, soft, evenly balanced image across the screen.
For the money, this is a superb buy and I am well chuffed with it. If you are in two minds about buying it for gaming, just go for it. You can upgrade your graphics card later this year if needs be.