Likely destined for a refreshed version of the 21.5-inch iMac, which currently does not have a Retina display, the new El Capitan beta references Mac support for a new 4096 x 2304 resolution Apple-made display panel. While the larger, more expensive 27-inch iMac is offered with a 5K display, it could make sense for the smaller screen to be upgraded to a 4K resolution.
Apple will continue to retina-ize its lineup, and the 21.5-inch iMac is ripe for the taking.
The 27-inch iMac with retina 5K display starts at US$1999, and the non-retina $1799. My guess is a 21.5-inch retina will start at $1499, which happens to be the starting price (at the moment) of the top-end 21.5-inch iMac.
A 4K 21.5-inch iMac would sport a resolution of 205 ppi. Though slightly less than the 218 ppi on the 5K 27-inch iMac it would hit a nice sweet spot in terms of price for those wanting a 4K all-in-one with a decently sized display.