The current 27" iMac's display is 2560x1440. The important thing about 5K is that it keeps the screen real estate of the current iMacs but gives all of the benefits of retina. If the new iMac's display was 'only' 4K, the screen real estate would be the equivalent of the 21.5" iMac (i.e. 1080P). I'm hoping for 5K and SSD as standard.