In a configuration file from the developers preview of Mac OS X Yosemite it has been discovered that extremely high supported resolutions were present pointing towards a brand new iMac Retina Display.
The first of these resolutions indicates in hex ’00001900 00000e10′, hence a resolution scaling of… 6400 x 3600 (probably a 3200 x 1800 HiDPI).
It continues and there are 5760 x 3240 (2880 x 1620 HiDPI), 4096 x 2304 (2048 x 1152 HiDPI), etc.
(There are other resolutions, just do the conversion from the hexa)
Bear in mind 4K resolution is usually 3840 x 2160 so these numbers are on the high side there. If we were to see these retina display iMacs, the most important question would be how much and when. Apple has had a history of keeping their refreshes similarly priced to the device they are replacing so fingers crossed it will be close to the current iMacs.