Speaking about graphical settings, i tested GTX 1080ti, changed only GPU (old one was GTX 770). Turned on every feature and settings to ultra, and after comparing benchmark results i guess i did right thing that didn't bought 4K monitor. Even 1080ti can't handle stable 60FPS with all shit enabled in most graphically demanding games.
For example, Deus Ex: Mankind Divided (Ultra settings, MSAA x4):
GTX 770
Average: 23.6 FPS
Min: 7.9 FPS
Max: 30 FPS
GTX 1080ti
Average: 50.1 FPS
Min: 7.9 FPS
Max: 62.2 FPS
Rise of the Tomb Raider (Ultra, SSAA x4):
GTX 770
GTX 1080ti
Far Cry Primal
GTX 770
Average: 29 FPS
Min: 38 FPS
Max: 62.2 FPS
GTX 1080ti
Average: 73 FPS
Min: 47 FPS
Max: 90 FPS
System specs except GPU remains the same, CPU i5-3507k and 8gb DDR3, DX11, 1920x1080. Not sure if CPU and memory are bottleneck in this case, but personally i just got confirmation that it's not the right time to upgrade for 4K and better wait another 2-3 years for new GPU and monitors generations.