4k Upscaling. Does it look really bad?

Hello,

I want to get a 4k monitor but even with a gtx1080 you only get an average of 30fps.

So I would use actual 4k resolution for more strategy games.

Whereas I'd use QHD or FHD to get a better FPS for more precise and smoother multiplayer.

The question is though when I lower the resolution. How horrible would the upscaling look?