I was running a 3 FHD (each 1920x1080) Monitor setup for years with Qubes 4.0.3 and a RX480 8GB.
My new Dell U2720Q has Native Resolution : 4K 3840 x 2160 at 60 Hz
Replaced one of the FHD Monitors who was also running via Display Port, i can see 4k Resolution at 60Hz but Monitor keeps going on and off very very often.
I did try,
qvm-features dom0 gui-videoram-min $(($WIDTH * $HEIGHT * 4 / 1024))
qvm-features dom0 gui-videoram-overhead 0
qvm-features dom0 gui-videoram-min 64800 qvm-features dom0 gui-videoram-overhead 0
but 4k kept blinking after reboot.
I also tried putting some into overhead, though it did not change anything, wiki is kinda vague from a novice standpoint.
Switched to FHD Resolution the Blinking Stopped but it is kinda blurry, that was yesterday night.
Decided to look later into solution, had to do some work on Qubes PC first, now after 4h of sitting in front of it and right when i had the time to look into this Display problem the new 4k Dell started randomly blinking even though it was on FHD.
My current guess is to look into the DP cable and replace it to get 4k running.