The 21.5" iMac with Retina 4K display has gun under the knife over at iFixit and of course there are a handful of tiny upgrades to the iMac as you would expect. However there isn't anything dramatic under the hood that Apple's iMac webpage hasn't already covered. But if you're a geek at heart, you could learn about the individual part upgrades here.
Of course the number one feature that's notable is the new 4K display. Interestingly iFixit actually doesn't cover the specs of the display in any way, so if you want to understand what a 4K display will actually deliver you'll have to rely on Apple's iMac Design webpage found here. It's there where Apple describes the pixel count, the wider color palette and the incredible technology that makes this display so remarkable – such as adding a more advanced timing controller, organic passivation, photo alignment and compensation film.
With all that said, in C/net's review of the new 4K iMac, Dan Ackerman stated that you're not really going to notice a lot of the advancements unless you're a photo or video professional dealing with very high end work every day. I'd have to agree. When comparing a standard iMac vs the 4K iMac I couldn't see the difference for day to day work or even photos – though I'm sure some will claim otherwise.