I’m sorry if this is a duplicate question, but nothing I do seems to work. I my main 1920x1080 monitor (DP-2) and a 3840x2160 monitor(DP-0) to the right of the main. Everything on the 4k monitor is tiny, I’ve tried every combination of xrandr commands, but nothing seems to achieve the desired outcome. All I need is for the 4k screen to have 200% scaling, yet I can’t get it to work. I’m also using X11 if that makes a difference.
Did you check: Global scale is not working - #4 by TriMoon 
It might be related, or even solve your use-case…
Global scale isn’t the problem. If I change the global scale to 200% to make the 4k monitor look correct, the 1080p one becomes unusable.
Ok just worth the try to look into that setting i thought.
Another option might be to manually configure the resolutions used by the particular monitors in your x11 configs, instead of letting it autodetect using EDID…
fe. only list the resolution you want for the 4k monitor and let the other stay on auto.
Examples:
In the past I tried this too. Xrandr I thought could do it on KDE.
But I failed, and explained it to myself “No works with Nvdia+X11”.
Would be interested if there is a solution too.
It auto-scaled to your liking when I once tried Fedora, or maybe it was Nobara. It was on Wayland. But then the whole desktop would crash soon.
If you can run Wayland, by chance also w/o an Nvidia card. You could make it, simply in KDE settings, but I’m not sure.