Hi,
I just thought I pitch in since I have been running a 4K monitor for quite a while now, and recently tested a newer model as well.
What you need to keep in mind is, that your computer will have to work 4 times as hard as before (assuming you had an "HD" display, 1920x1080), since you will now be editing your image on four times the resolution (3840x2160).
I have a MacBook Pro 13" with the Retina display, and when I ran it on a 4K screen (actually a "Cinema 4K", the LG 31MU97), I noticed quite a difference in performance.
Another thing to keep in mind is, that a 31" 4K screen is quite big. This means that you may have trouble with color-shifts, bleeding etc. when viewing just slightly from the edge. The LG monitor I tested was so bad, even looking at it straight on was not producing an evenly lit image. You don't notice this so much with bright images, but anything dark will be annoying.
So my suggestion is, if you want to go for 4K, make sure you have a very powerful computer to run it, especially with lots of memory as was mentioned in this thread already.
And when you buy a monitor, be sure to test them before you buy, or at least make sure you have a 30 day satisfaction guarantee.
I have been running an IBM T221 since a few years; it's a dinosaur, it was rather an experimental monitor and does certainly not have amazing color and brightness as the newer models today. Nevertheless, I decided to stick with it until Eizo comes out with their 24" 4K monitor (for which I will also have to sell a few cameras...).
My observation was, that images look fantastic on the IBM due to the high DPI. At 22 inches, the DPI is 204, which is in my opinion quite a noticeable difference when compared to the LG, which is 31" and has about 150 DPI.
Having said that, the iMac 27" Retina may be just right for you—more than 200 DPI, lots of power etc. I have however heard some photographers complain about it's color accuracy, so I would definitely spend some time at the Apple store with some of your own images to test it out thoroughly.
And last but not least, Just in case you became curious about the IBM monitor, forget about running it on Windows, I spent countless hours trying to configure it. I doubt it will run on Mac, unless you have one with four monitor outputs. I am running Linux since many years, and got it to work there perfectly. It was not an easy task however...