Apple's larger, 27-inch iMac got a big upgrade with a 5K display back last October, but the smaller desktop has since remained with a lower resolution screen.
Based on OS X code, it's a safe bet the 21.5-inch model will have a display resolution of 4096 x 2034 pixels. That works out to a pixel density of 218 pixels per inch.
An open question is still the price of any such configuration. The larger 5k model starts at $1,999 while the current 21.5-inch iMac with standard screen begins at $1,099.
I'd expect Apple to continue selling the lower resolution models -- but with improved internals -- at the same price. The new 4K iMac then would fit in nicely around $1,299.
One thing I don't understand about the current 5K and anticipated 4K iMac is this though: Why hasn't Apple made them more appealing by getting them certified to work with 4K / UHD content from Netflix, Amazon and others?
I recently purchased a Vizio 4K TV to watch higher resolution programs and while I was shopping around, I realized that not all 4K sets support the content. You have to make sure that the product you're buying is supported.
Clearly, a 4K or 5K iMac has the chops to show such content.
If Apple could the desktops added to the support lists, it might further entice buyers to pay the extra premium and get the higher resolution desktops.