I find it so strange that most people still talk about their monitors' resolution in terms of pixel counts. Oh, you have a 4k monitor? Well, if that's a 43" 16:9 4k monitor, then that's pretty close to 100 dpi, so you basically have a big-ass traditional ("lodpi") monitor. Whereas if it's a 24" monitor, then it's a smallish hidpi screen. Rather than pixel counts, I find it much more helpful if you tell me the size in inches and the DPI.
This is related to one reason why I'm not so crazy about 4k: At most of the usual screen sizes, they occupy an unpleasant middle ground between a traditional screen (~100 dpi) and a 2x hidpi/retina screen (~200 dpi).