I wouldn't even consider 4K before I got my 1080 Ti. That card makes it a real possibility for me now. Although I can't go back to 60Hz, so I have my eye on the ASUS ultrawide curved 3440x1440 with HDR @ 200Hz.. Even if there was a cable with enough bandwidth to allow that in 4k it would cost a fortune.
I'll probably stick with my 2560×1440/144Hz monitor a bit longer.