4K at 24-28" will result in text and interface that is either too small to see, or too big to be useful, because of how macOS scales for 1x (standard) and 2x (Retina) displays. macOS can do either 1x or 2x, and 27" at 4K (for example) is about 1.5x.
Connection-wise, using 4K at 60Hz (you want 60Hz - 30Hz will appear laggy) will require using the Thunderbolt port as the HDMI output on the MBP will not do 4K at 60Hz.
Basically, 4K is not a good idea: you're either running it too big, making the desktop feel cramped, or too small, hurting your eyes and ruining the Retina effect.
I would recommend looking for a 27" 1440p monitor instead. There are lots of these around in your price range. When you next update your Mac, you can then look at 4K displays (in the 21.5" region) or 5K displays (in the 27" region) as these are scaled correctly. The prices will also have come down by then.
Hope that helps.
Honestly I thought the macOS scaling was only for built-in Apple displays - and thought only 1x and 2x were possible for externals. Still, the scaling is sub-par. It is best to stick with 1x or 2x.Huh? That's not how hiDPI scaling works in macOS. You can choose a 'looks like' resolution that is somewhere between those two extremes and gives you the size/space ratio you wish. There is a bug with some configurations in macOS Sierra but it doesn't affect all configurations. In general 4k monitors work fine with macOS.
Honestly I thought the macOS scaling was only for built-in Apple displays - and thought only 1x and 2x were possible for externals. Still, the scaling is sub-par. It is best to stick with 1x or 2x.
https://twitter.com/marcedwards/status/795602525772578817
https://twitter.com/marcedwards/status/795427805345366016
It's a matter of preference for sure, but personally I wouldn't be investing in scaled or 30Hz 4K.I'm really really picky about my displays, but I've been running a scaled resolution on my rMB for 18 months now and it's more than sharp enough for me. When I connect it to my 4k display I also use scaling there and it seems to work well enough. I would expect at 27" 4k display would still be sharper even scaled than a 27" 1440p display natively. You start to take a bit of a performance hit with the rMB because it's low on GPU power to begin with but for this user with a newer MBP, should work fine.