With a Titan card from nVidia, maxing out at 20 FPS on Unigine, it's pretty depressing. We need to see a 2 to 3x fold performance increase in graphics cards for gaming to be realistic on 4k screens.
Depends what you call decent. 15 FPS(minimum) at medium settings in Crysis3 is hardly what I'd call decent for the best graphics chip out there. I'd say it's playable at best, but far from being where you want to be.
No, they both had 24 FPS minimum at medium settings.
Minimum really isn't all that interesting, though. A single low spike won't ruin a gameplay session. The average FPS was 42, the low spikes were few and far between. It's absolutely playable - that's the entire basis for which the settings were picked btw.
Average FPS of 42 is OK, but in any serious FPS you'd want to have 60 FPS constantly at least. Any number lower than that and your shooting accuracy decreases greatly with the framerate.
So, playbable, probably, but enjoyable, not so sure about that.
Unigine is not a game and it does not in any way reflect real world game performance. The Heaven benchmark has proven time and time again to be entirely irrelevant, due in no small part to it's insane usage of tessellation.
For the majority of games 4K won't pose an issue whatsoever with a modern high end video card.