How do I get 1920x1080 resolution to fill the whole screen on a 4k monitor rather than it just getting smaller?

[RomanWolfEater]

I just got this game and even my system has 4K resolution it always runs poorly, So I want to run it in 1920x1080 but either it be in borderless window mode or full screen the game window itself gets smaller in the corner of my screen. I can see the game still and do things and it runs better, but now it is so tiny that I can't see anything. What do I do?