The following settings work for me:
In Display Settings:
Scale: 200%
Fractional scaling: OFF
In nVidia settings -> xserver display configuration -> advanced
ViewPortIn 5120x2880
ViewPortOut 3840x2160+0+0
Panning: 5120x2880
Save to X configuration file
Ubuntu 20.04, nVidia 1060, Dell 2715P 4k
The following settings work for me:
In Display Settings:
Scale: 200%
Fractional scaling: OFF
In nVidia settings -> xserver display configuration -> advanced
ViewPortIn 5120x2880
ViewPortOut 3840x2160+0+0
Panning: 5120x2880
Save to X configuration file
It gives me 150% scaling and it survives reboot