firsttry51, I think I found the variable for the screen size. If you still want to give the 4K a shot the variable is:
store.persistent._virtual_size[0] = 1920
store.persistent._virtual_size[1] = 1080
Just change the variable using the console command and see if the image fit the screen.
Command for 4k:
store.persistent._virtual_size[0] = 3840
store.persistent._virtual_size[1] = 2160
Unfortunately I don't know where in the game files "virtual size" variable is stored so the change is not permanent but should be good enough to test the 4K image with.