There's an argument that 4K is on the excessive side for a monitor, but then I'm sure the same conversations were had when 720p and 1080p resolutions were cutting edge. If you're after more pixels per inch, then it's hard to go past Dell's P2715Q. It usually sells for $889, but right now it's $711, delivery included.
Spotted by OzBargain's madmouse58, the deal doesn't have an expiration date printed anywhere, but you can be sure it won't last long -- perhaps only for the weekend. For those not up to scratch with the lingo, 4K works out to be 3840 x 2160 -- plenty of desktop space for all your day-to-day needs.
The display also boasts grey-to-grey speeds of 6-8ms, 350 cd/m2 brightness and a 1000:1 contrast ratio. Connectivity includes DisplayPort in/out and Mini DisplayPort, as well as HDMI and four USB 3 ports.
If the P2715Q is a bit rich for your blood, the more reasonable 25-inch U2515H can be had for $415. You miss out on 4K, sure, but 2560 x 1440 is still fantastic and you'll save yourself close to $300.