iFixit Gives Apple’s 4K 21.5” iMac Display the Tear Down Treatment
iFixit took the 21.5-inch Retina 4K iMac apart.
21.5-inch Retina 4K iMac
Teardown Finds 21.5-inch 4K iMac Has Soldered RAM and 27-inch iMac Supports 64 GB
The 21.5-inch Retina 4K iMac's memory is soldered, so you better order as much RAM as you can when buying the machine.