Currently, no pair of cards can deliver a consistent 60 fps at 4k. The 1080 does 45-50 on Witcher 3 so it is still coming up quite a bit short.
The other problem is that, at least from what I have found, no monitor comes with the necessary bandwidth nor cable connections to deliver 4k at above 100 hz. So far, haven't seen 1 monitor of any kind with anything newer than Display Port 1.2. The 1080 comes with three DP 1.4 ports which **can** carry the necessary bandwidth at 4k to support high refresh rates.
Buying a 4k monitor now will result in a huge case of Buyer's Remorse for many folks as the value of their investment will drop like a rock once DP 1.4 models come out. I see 4k finally gaining some momentum once DP1.4 equipped models (w/ G-Sync and ULMB) start shipping. It will get another boost once the 1080 Ti drops which should be able to maintain 100 fps in 2-way SLI and allow folks to use ULMB w/o issue.