It's so strange that when you put more fps to have a smoothier experience and faster reaction you increase lag so much.
That because his PC cannot keep the same FPS at 4k ultra, as it does at 1080p.
The more fps a game renders at, the more often it "updates" things on screen, and as such, the instance it notice a "key press" happends more often too.
Thats why the input delay rises when hes doing testing at 4k, vs 1080p.
The 144mhz is just what hes asking the monitor to output at.... if his PC cannot keep that frame rate at 4k, then thats to be expected.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071