Some time ago I got a laptop with a 4k display. I am not using that one with
external monitors, but I still may be able to help a bit. With earlier versions
of Plasma, scaling was pretty much not working. I was looking at tiny text all
This changed at some point - I think when Plasma 5.6 rolled around, but not
exactly sure. Anyway, what I did was switching my scaling (System Settings >
Display and Monitor > Display Configuration > Scale Display) back to 1, so not
scaling up. On the other hand I set my font DPI (System Settings > Font > Fonts
> Force fonts DPI) to 96 (after ticking the box of course). Since that version
of Plasma, all interfaces appear to scale nicely in response to this. Firefox
and Chrome apparently respect that setting too (although I may have played
around a bit with settings there, cannot remember).
Hopefully, the fonts DPI works across displays, given that Plasma knows the DPI
of the monitor...?