Along with a spectacular display, the 21.5" Retina 4K iMac has some new age specs:

3.1 GHz quad-core Intel Core i5-5675R (Turbo Boost up to 3.6 GHz)

8 GB of 1867 MHz LPDDR3 RAM

Intel Iris Pro Graphics 6200

1 TB (5400-rpm) hard drive, configurable up to 2 TB Fusion Drive or 512 GB of flash storage.

802.11ac Wi-Fi + Bluetooth 4.0

Shiny new EMC number, 2833 (same ol' model number A1418)