In terms of FPS Gaming (all I really care about and do) I found 1440p to be perfect, more so because of the type of FPS games I play (Squad, Post Scriptum, Escape From Tarkov and Arma 3). These games have alot of ranged fighting, and flinch type CQB. They are by triple A games standards, very hard to run decent FPS on.
I found the difference between 1080p and 1440p significant. And 1440p and 4k less so.
The issue is keeping the frames high, and the input lag low. 4k hits frames pretty hard, and generally speaking the monitors available have a higher input lag. Above 60Hz refresh in 4k is expensive, on a decent screen.
If you are not a competitive FPS gamer, and can drive playable frames in 4k - do it!