According to the folks at 9to5Mac, they have spotted within the code of OS X El Capitan that there have been some references made to a smaller Retina iMac. More specifically, the beta of OS X El Capitan has revealed a new 4K resolution in the form of 4096 x 2304. Granted there is no mention as to which iMac would be getting this display, it would make sense that Apple would use it for the smaller iMac rather than the 27-inch model.
Apple hasn’t really done a huge refresh for the 21.5-inch iMac. In fact the last time we saw an iMac “refresh” was when Apple lowered the price of the 5K iMacs and introduced a lower-end model for customers who might not have been able to afford the higher-end models. We can only assume that the 21.5-inch models would be even more affordable but this is just speculation for now.
No word on when Apple plans on unveiling these new computers, but chances are we might get to hear about them in the later part of the year during Apple’s iPad event, which is also when the company announces new computer refreshes like they did last year.