So we've been using the term "4K iMac" and "Retina iMac" interchangeably, but in reality they are different things. A 4K iMac at 3840x2160 is an improvement over 2560x1440 but the pixel density would only go from 109ppi to 163ppi. That's a nice to have, but it's not the life changing visual experience the Retina MacBook Pros or iOS devices had after being truly pixel-doubled to Retina. Plus, a 4K 27-inch iMac would be nearly unusable at native resolution so you'd end up running it at "Best for Retina" which would simply be 1920x1080 HiDPI mode...essentially cutting the perceivable screen real estate vs what we have now. You could then scale the display for more space but you'd lose some of the crispness you bought it for. We're still very far off from 5120x2880 which would be a truly Retina version of the current 27-inch iMac so anyone holding out on buying a 27-inch in hopes for a Retina version may want to reconsider.