Did you go into your settings? GeDoSaTo makes it possible, but you have to set the resolution (4K) in the video settings of the game.
I uhh, can't believe I didn't think of this.
I did also enable some sharpening and color pallete post.fx stuff as well, but that didn't seem to work.
I just tried it, and set the resolution to 4k and it said it was downsampling to 1080p. So it seems to be working.
totally destroyed my FPS though, holy shit. I wasn't expecting that. from ~120 to ~25-30 in gran soren. damn.