Hello. I've always worked with three monitors, all three full hd. Very comfortable. The main monitor (the one at the center, those at the sides are just for convenience) left me and I changed it with a brand new 28 "4k ... disaster! Managing the monitor with so much difference of resolution is still not feasible I think. At least from kde (but I do not think it depends on him [or her?]) There is no way to set scaling for single monitor, but it's a global setting. So i find that the two full hd have become unusable as gigantic , And the 4k still has it all very small.

I tried to raise the font dpi from 96 to 146 and it's better ... but it's still not very usable.

Does anyone have any experience about it?

I can't speak to your particular situation as I only have 1 4k monitor, but I think many DEs fail to do this. I am not 100% sure, but I think this may be a limitation of X and Wayland may resolve in the future?

Any way I think a hack exists, but it isn't perfect. https://www.reddit.com/r/kde/comments/5 ... e_monitor/

Personally I would just drop the < 4k monitors and sell them to purchase another 4k if you really need the screen space. In Linux it currently just isn't worth the hassle. I could be wrong though, maybe some one else knows a better method.

Also maybe look at this gist - https://gist.github.com/wvengen/178642bbc8236c1bdb67