I run 4k at the moment on a predator XB1 monitor (nvidia sync). I'm using a single EVGA 1080ti FTW3 and it's been plenty. That's also with a I7 8700k overclocked to 4400ghz. My monitor is only a 60hz so that's where I cap on fps. That's enough for me because I don't really do FPS, mainly play single player RPGs, etc.
I assume if you're looking at doing FPS multiplayer, you'd be better off at 1440, running 144hz. Again, the above listed setup could likely do that just fine as well.