A 4K 21" dispaly wouldn't need a custom display controller. 4K is inside the realm of DP1.2 bandwidth. If just ease up to a 24" 4K iMac it is down right almost trivial ( as Dell and others sell a 24" 4K monitor already
http://accessories.us.dell.com/sna/productdetail.aspx?c=us&l=en&cs=19&sku=860-BBCD )
5K iMac makes more sense short term though because there is a $2,499 price hole in the iMac - Mac Pro spectrum that Apple left open when they moved the base Mac Pro up to $2,999. It isn't very surprising at all that Apple has stuffed an "iMac Pro" into that hole.
When 5K ( and 21-24" 4K ) prices fall so that Apple can slide the smaller 4K screen at the current iMac price points then they'll Retina across the line up. In mean time it would highly clutter the line up if started overlapping 'regular' 27" iMacs with a string of hiDPI 21-24" models.