I would not call my advice "out of date very soon" right now. I have seen reports of OS X 10.9.3 supporting 3840x2160 at 60Hz. However, I could not find any information on how this resolution will be supported at this refresh rate. Will it be supported with HDMI or Thunderbolt, or both? This information is important for those wishing to buy a new monitor. So, I would wait for the 10.9.3 release and for further information prior to buying anything.
I have not seen any of these monitors in person and I cannot tell how good they are. However, I have seen some mixed consumer reviews.
The first to be released was this Dell, as far as I am aware of:
http://www.amazon.com/Ultra-Monitor...UTF8&qid=1399658266&sr=8-1&keywords=3840x2160. A bargain for US$ 600.
There is also this Samsung monitor selling for less than US$ 700:
http://www.amazon.com/Samsung-28-In...UTF8&qid=1399658266&sr=8-2&keywords=3840x2160.
There are additional 28-inch 4k monitors in the pipeline, including offers from Asus and Lenovo. They all appear to use the same panel.
I have not yet seen expert reviews about these monitors, but I found this:
http://www.tested.com/tech/pcs/459878-dude-dont-get-dell-4k-monitor/
Not very encouraging, huh?
There are, of course, some great 4k monitors, but they are expensive. The 24-inch from Dell (
http://www.amazon.com/Dell-Computer...UTF8&qid=1399658266&sr=8-3&keywords=3840x2160) is IPS and looks better or, if you want something bigger, you may go with the 31.5-inch (
http://www.amazon.com/Dell-UP3214Q-...UTF8&qid=1399658266&sr=8-5&keywords=3840x2160).
In any case, you should always check the ports and the possibility of the specific monitor you choose displaying 4k at 60Hz. A 30Hz refresh rate may feel like a downgrade from your iMac.
And note that these 28-inch TN monitors are also inferior to the monitor in the iMac, which is an IPS screen.