This obsession is really, really dumb. The current generation is 109ppi which at a normal viewing distance pretty much is Retina.
No and it's not even close.
rMBP is 220 ppi. For the iMac to be retina at 109 ppi, you'd need to sit at a distance twice as long as you use your macbook, which is not the case. I use my Macbook at an approximate distance of 60cm. There's no way anyone uses an iMac at 1.2 meters. At most it'll be around 70-75cm's. So you need around 200ppi for a retina iMac. And that's feasible today. Just cram the 4K into the 27 inch screen and you are done.