When running the game in windowed fullscreen mode, it looks like its impossible to get the game world to render at anything higher than 2560x1440. The only way I can run the game in 4k resolution is to use regular fullscreen mode, which is a bit of a nuisance because of how it operates.
I've even tried editing the GameUserSettings.ini to force it to run at 3840x2160 in windowed fullscreen mode, but the game ignores it and reverts back to 2560x1440. I tried making the .ini file read only, but the game still ignores it.
Also when swapping resolutions around trying to fix this, I noticed sometimes the game will set the 3D render percentage below 100% sometimes (for example one time i swapped from 1440p to 2160p and it kept the 3D render scale at 66.6% (akk 1440p). I could fix that, but it seems like a bug.