"4k monitor": 3824 x 2160
"5k monitor": 5120 x 2880
That's a lot of pixels.
It might be a good idea to be skeptical about spending >$1,500 on a 27-inch monitor in Q4 2014. It's difficult to notice any pixelation on a 27" screen at a resolution of 2560x1440, so clearly the reason to upgrade to 5120x2880 is for the extra screen workspace. But unless you have very good vision, you're probably not going to be able to read text at 5120x2880 without zooming. What's the advantage?
For $1,000 you can buy two 27" 2560x1440 monitors, which is a huge amount of workspace. Also, a single $300 midrange GPU can drive both monitors at full resolution. A couple years ago, that was cutting-edge tech, but it cost ~$2600. Also, two monitors offer a better user experience than one monitor, since window management is a bit easier.
Would anyone mind explaining whether the pros of a 5k monitor outweigh the hefty pricetag?