You do know how much "4k" screens are right? I think there are currently only one or two out and they are both around £4k alone! The panels the 27" iMac uses can be had in monitors for anything from around £350 (rubbish ones) to around £600.
That'll be why they haven't released a "retina" iMac...
Not particularly, it's not sensible to look at the retail price of 4K displays and use that as the basis for why Apple hasn't started using them yet.
Additionally, 4K wouldn't or shouldn't be considered "retina" on a 27" monitor at all.
If you use that logic, 27" 2560x1440 displays vary so massively in price that it's evident that retail price is not a reliable indicator.
For example, if you lived in Korea, 27" 2560x1440 monitors are around £100-120, and you could go and buy one of them for that much from a local Korean store. They use the same panels as Dell's 27", Apple's iMacs and other displays, despite being a fraction of the cost.
4K itself is just a number, and it's not a particularly big one either considering how far mobile display technology is ahead of desktop monitor technology.
A 4K 27" monitor would frankly look like arse compared to one of the latest 1920x1080 phone displays on the market.
So there's a lot more to it than simply an expense, as Apple had Samsung make the 2880x1800 displays specifically for the macbook retina models, and as far as I know, only Apple are buying those displays.
So Apple has the ability to have such a display made, it's just a case of whether it's viable yet or not, but 4K is not enough for a 27" display to be "retina".