Screens that exceed 100ppi are a novelty... Well unless you need more space for multitasking.
As a photographer I can say that 4K would be nice if it was a 42" 16:10 10bit S-IPS panel... Then the pixels would actually be beneficial when looking at pictures. Till said monitors arrive if my PA301W Nec at work died I would be ordering another asap. I have a friend who's a sound engineer who is more interested in purchasing 2 21:9 panels at 3K over a 2 4K 16:9 panels.
For video and games the 100ppi affect kicks in and makes zero difference... Meaning that 22-24 = 1920x1080 and 27-30 = 2560x1440/2560x1600 are ideal and resolutions and higher than those on the same size panel offer almost no visual improvement in video or games.
If I were to get two 28" 4K panels and have one running a game at 4k and one running it at 2560x1440... You would not be able to tell them apart.
I'm sure there's diminishing returns the higher the ppi gets where less improvement will be noticed with higher ppi but I would much rather look at 400+ppi (like my note 3 screen well its close to that at 385 ppi)display then a 100ppi or less display I defiantly don't feel like 100 ppi is where the diminishing returns really start to hit its closer to the 200-300 ppi range imo.
of course the distance also makes a difference. if you were to keep your note 3 as far from your eyes as you would a 24" monitor the effect would be closer to what you would see 4k on a 28 inch screen (a completely rough and in no way scientific estimation).
there's a lot of mixed feelings about 4k monitors but the general feeling is that they are useless at 24" and only start becoming feasible at 27" or larger screens. personally, i won't be convinced until i see it with my own eyes, and i haven't yet seen a 4k display to make any sound judgement.
For 4K I wouldn't even bother with smaller monitor sized screens I'm personally interested in 4K for the potential to have a large t.v sized screen say a 50" t.v on my desk up close but with relatively decent ppi so it doesn't look so pixelated that close like a 1080p set would.