firsttry51, I think I found the variable for the screen size. If you still want to give the 4K a shot the variable is:the renpy files that need replacing are only like 20mb, it's just code stuff.
the 4k images didn't fit to screen properly for example
ingame
View attachment 3186089
outside game
View attachment 3186090
i don't really know much about renpy to make it fit properly
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.