I also got a 4K 28" monitor that was discounted a few months back - in my case an Acer. At the time, I was running a fairly new 27" Acer monitor and two smaller, older LCDs. I discovered that my system couldn’t drive 1 4K and two other devices so I now just use the two large monitors and am generally quite content with screen real estate.
HOWEVER … although I can run 4k on the one, having one monitor at 1920x1080 and the other at 3840x2160 makes little sense - either the menus are too small on the 4k monitor, or scaling everything means things are too large on the 2k monitor. Besdes, why scale things when you have high resolution - isn’t the point to use the high resolution?!
So I’ve ended up running both monitors at 1920x1080, and Kubunutu 18.04 does this very nicely.
It can be nice to have a much larger proportion of the screen for my pictures when editing (I use darktable), but the text in the sidebars becomes worthy of squinting. So darktable is certainly a piece of software that would benefit from being adapatable to 4k monitors.
Occasionally I ramp up to 4K on the new monitor for OpenTTD (Transport Tycoon), but even then I usually prefer spreading my game across two monitors of the same resolution.