Absolutely no desktop environment can properly handle having a 4k display and a 1920x1080 13" laptop panel at the same time if you need to keep switching between with-monitor and without-monitor.

Yes they all claim to have "hidpi" configuration, and those configs mostly work if you're always using it in the same setting, but as soon as you start having to disconnect/reconnect your monitor to take the laptop somewhere else your troubles start. Applications can't properly handle changing DPI on the fly, there are often apps that get completely confused by their changes: they either show tiny fonts (e.g., cinnamon taskbar launchers), or adjust the font sizes but not the spacing between lines so you can only see half of the characters (hexchat) or whatever. It's horrible.

It makes me want to write my own desktop environment, or fully switch to text-mode-only apps.

I have a Samsung monitor and I'm not super happy with its colors. I always enjoyed the colors from Dell monitors. But if I ever buy another monitor I'll probably look at its response times.

Another option - Dell UltraSharp 27 4K USB-C U2720Q

I find my 31.5" to be almost a bit too bit for dev. So far I like 2 x 24" 1080p's.