Earlier today as rumoured, Apple unveiled a new 21.5-inch iMac with 4K Retina Display as well as updating the whole smaller iMac range with the latest Intel Broadwell chips. Apple is offering a 3.1GHz quad-core i5 with the option to upgrade to i7 chips.
Previously the only iMac that was offered with a Retina Display was the the larger 27-inch 5K iMac, which is still the most powerful iMac that you can buy. With this new panel, the resolution has been dramatically increased to 3840 x 2160, it also has 25% colour.
The 21.5-inch iMac 4K starts at £1,199, although the less expensive model for £899 is still available, but lacks a Retina Display.