I'm on my 4th (!) 4k monitor, and think I have found the sweet spot - it's a 48 inch curved 4K Samsung JS9000 TV that supports 4k @ 60hz with 4:4:4 chroma over HDMI 2.0.
At that size the pixel density is identical to a 24" 1080p monitor and the slight curve means it fills your field of vision with significantly less distortion at the edges.
Unfortunately the stand isn't adjustable, so I replaced it with a set of adjustable 'feet' off Amazon that bolt to the wall-mounting holes. The screen sits about a centimeter off my desk - just enough to run keyboard & mouse cables under, while keeping both the top and bottom of the screen at comfortable viewing distances.
The overall result is _glorious_. I have so much room I can just drag windows over to whichever part of the screen I feel like staring at, while having tons of room at the top for widgets and music players and such. I can run games in windowed mode alongside web browsers and everything's readable and playable. I can have pages and pages of code spread out everywhere and it's all marvelously usable.
In comparison, my other 4k experiments -
1. Seiki SE39UY04 39" - Terrrrible lag, bad color, 30 hz only. The 30 hz is not itself the killer (30 hz on the JS9000 is only mildly irritating), it's the 100+ millisecond delay that makes it infuritating. Sold it for $100 after trying and failing to adapt.
2. Samsung 55" UN55HU8550 - Great except for the lack of 4:4:4 chroma, and 55 inches is just -too- big to use as a monitor - especially without the curve. I still use this one as my main living room monitor & TV.
3. LG 40UB8000 - Almost good enough. No 4:4:4 chroma, slightly buggy firmware, too much color distortion at the edges when used at monitor distances. Pixel density is about the same as a 27" 2560x1440 monitor, but in practice you'll be squinting as it needs to be placed farther back on your desk in order for the sides of the screen to be usable.
Samsung does make a cheaper 48" 4k screen (the JU6700) which I believe supports 4k@60, 4:4:4 chroma - it would probably be just as good as the JS9000 for most purposes, with only slightly reduced color quality and a few less bells and whistles.