There were a few caveats that prevented The Wire Cutter from recommending any monitor, but some of those don't seem to apply as readily now as they did at the time of last edit.
I got the Dell UP2414Q - a 24" ~4k monitor, making it around 183 ppi. The primary issues The Wire Cutter brought up in their rundown (of this display and generally) seemed to be the following:
- price (in this case $1100)
- the resolution being too high for the screen space
- poor refresh rates (30Hz in this case)
On the first point, you can now get it for around $750, maybe less, in the US. The second point was somewhat platform-specific, but on OS X you can tell it to display in HiDPI mode and it'll deal with the resolution in pretty much the same way that it deals with a retina Macbook Pro's 2880x1800 display. Namely, it'll scale everything at 2x. This is idyllic. If you have an rMBP, imagine that kind of sharpness across a 24" display and you're in the right ballpark.
The last point, about refresh rates, is trickier. My understanding is that this display has no trouble displaying at 60Hz once you enable it, but I have this display hooked up to a hackintosh, so myriad driver quirks make it impossible for me to say definitively whether the video card, the monitor, or the whole universe is conspiring against my success in trying to get it to display at 4k@60Hz. I think it's the buggy nature of the solution I came upon, which I could only describe as "not working in varying degrees". I'm still looking around, but I'm not optimistic for a real fix to get 60Hz for another month or two, if I'm lucky.
If I had a 2013 or later rMBP, I understand I could test it since that supports the DisplayPort 1.2 standard, but mine is from 2012. So I'm stuck testing with a desktop that shouldn't really even exist, heh.
So bringing it full circle, I recommend that you give waiting a chance. The Dell monitor I have has been fantastic, but not without its frustrations. If you can wait 6 months (or better yet, a year) I emphatically believe that the long term solutions to these kinds of displays will have shaken out by then. DP 1.2 and HDMI 2.0 will almost certainly be ubiquitous in video cards and probably in most laptops. It'll just be a lot easier to get things up and running.
That being said, if you're willing to put in some work and get frustrated a couple of times along the way, the reward is quite nice.