I haven't had this experience (MacOS, 4K monitor for 2.5 years)
> 2. If the 4k screen is on your laptop, good luck ever having a decent experience plugging in a 1080p monitor. Also good luck having anyone's random spare monitor be 4k.
shrug - 4K and 1080p seem to work together just fine for me. I've currently got a 27" 4K monitor and a 24" 1080p monitor both running off my 2015 13" MacBook Pro; the 4K is on DisplayPort (60Hz @ 3840x2160) and the 1080p is on HDMI (and it happens to be in portrait mode). I use all three screens (including the laptop's), and while the 1080p is noticeably crappier than the other two, it's still usable, and the combination of all three together works well for me. A couple of extra tools (e.g. BetterTouchTool) really help with throwing things between monitors, resizing them to take up some particular chunk of the screen, etc. - my setup's quite keyboard-heavy with emphasis on making full use of the space inspired by years of running i3 (and before that xmonad, ratpoison and others) on linux and freebsd.
> 3. Configuring my preferred linux environment to work with 4k is either impossible or just super time consuming.
That's a statement about linux and i3, not monitors. (And again, I like i3, but stating this limitation as if it's a problem with monitors not i3 seems... odd.)
