I have the best part of a decades experience with this. Until recently I was running 3 4K displays on my desktop. The real issue is with DEs and not Linux per-

I have the best part of a decades experience with this. Until recently I was running 3 4K displays on my desktop. The real issue is with DEs and not Linux per-se. Gnome handles 4K (Hi-DPI) better than most other DEs I've tried but it's still pretty horrid with Hi-DPI and non Hi-DPI displays mixed. My rule of thumb has been to try keep all displays at the same resolution. For desktop that's easy, for laptop; buy a laptop that's 4K (not always cost/battery effective), or turn off its display when using external (loss of screen real estate) or change the external monitor to 1080p (not ideal, loss of resolution).