From the reviews available right now I would say that the 1080 it is very capable of 4k gaming. It is true that it can't use maximum settings on all games at 4k, but you can get about 95% of the maximum settings on most games and still have 60 FPS.
Right now there is no other alternative, though, as the Radeon RX 480 8GB is comparable to a Geforce GTX 1060 6GB in most games. Of course the AMD Vega is supposed to be a bit faster than a Geforce GTX 1080, but that has not been verified yet.
Personally, I am running an EVGA Geforce GTX 1080 SC card on a 3440 x 1440 Acer X34 monitor and the GSync has to kick in due to frame rate drops frequently in Fallout 4 and Witcher 3. I do have all GPU settings maxed out, though, and the monitor refresh rate is 100 Hz.