A few weeks ago, I bought a Dell UP2414Q 24" 4K display. I had a 27" for a few years but found it actually too large. I'm mid-40s and don't wear glasses (yet) but my eyes every week it seems are headed that way. I digress. I bought the Dell 24" to match my retina macbook pro. It's a perfect match. And 4K on the 24 is basically like 5K on 27. I'm graphic designer as occupation and photographer as hobby. Colour and contrast on the Dell 24 are great. And it's matte finish. No glossy reflections.
All your apps must be really hard to work with though? There are scroll bars and text in the Adobe suite which currently can't be scaled (AFAIK) and they are already too small even on a regular display. At that pixel density they'd be a nightmare!
With Photoshop on Windows (but apparently not on the MAC or not for Lightroom on either) that is true, but I happily deal with it to edit, in realtime, essentially 8MP prints. And it's so awesome to be able to instantly view every photo you have without cost with the detail of an 8MP 24" print.
And text looks so much better too. Like right now, as I type, it's like I'm typing in a magazine or book and it doesn't look all computery at all. It looks like real print.
And images just look natural. They don't look computery. They just look far more organic and natural, more like looking at a giant magnified slide table or something, they don't have that blurry, pixellated so sort of subtly digital computery look so much now. And the 27" 14MP displays must truly be insane, wow.
Imagine an amazing photo coffee table book printed at 240PPI and then one printed at like 80PPI. The looks decent if still a trace soft and the other just looks a mess.