I'm using a 32" 4k monitor without scaling, even if my eyesight is not the best. I have no trouble at all with it. It's the more common 27" 4k monitors that have tiny fonts and need scaling. But 32", 4k it's fine at 100%!
As for 1080p, it's enough for most things. You mentioned the dpi comparing it to a 24" 4k monitor, but why would you need 24" for a 1080p monitor? Anything above 20" is a waste for 1080p.