A single 1080 ti handles 4K/60hz no problem on Ultra/High settings with no dips under 55 FPS. I've tested this with the following games...
-BF4
-WoWs
-WoW
-Witcher 3
-BnS
-Tera
-The Division
-Fallout 3/4 (30+ mods)
-Skyrim (100+ mods)
-Dying Light
-Planet Coaster
-Hellblade: Senua's Sacrifice (looks amazing at 4K)
Update: I do want to make a note that when I do utilize 4K I do have Nvidia Geforce Experience adjust the settings but they are a mix of mostly Ultra/High settings with Nvidia's AA usually.
There are others but I can't think of them atm. Even going with a 1080 or 1070 ti for 4K is reasonable if you don't mind lowering the settings a little bit. As for 4K/120hz or higher it's not expected for this generation of cards to be even close to that, heck even by Steam standards most ppl are still at 1080P.
For me 3440x1440P/200hz (releasing soon I think) would be my monitor of choice over 4K/120+.