Just continuing thread as for me it's only just speculation at this point. I have new the riMac and would still have preferred a smaller 4k choice as I like the reduced field of vision necessary that smaller panels provide.
I wonder if it be viable for apple and it's vendors to tool up production for a custom panel on such a low volume product ( as lcd fabs are having a difficult time meeting demand as it is ). 5k was in the works and announced earlier in the year, so was not custom panel. Apple was able to just drop it in existing 27" iMac other than ( TCON chip and AMD GPU ) they had to drive it with, hence Dell 5k is available as well. ( LG/Samsung ) might decide to produce smaller panels ( i.e. 21.5" ) as 4k adoption picks up pace, which then we may see 21.5 4k iMac. Its seems they start to announce new panels in spring and start production in fall. So maybe next 3/4th quarter.
As for pixel density, all displays are exhibiting various ones, ( iPhone, iPhone plus, iPads2/mini etc. ) and non retina 21.5" pixels ( 0.248 mm ) are still bigger than non retina 27" ( 0.2331 mm ) panels, while closer than 24", it still makes a visible deference. At the moment 23"/24" monitors are more popular with consumers than 21.5" so having a bit larger display might be a good option for apple as it continues to make progress in the PC space.
We'll see.