Actually I believe retina is a function of not just resolution but how far you sit from the display so if you sit close enough, it may not be.
Ah, my bad. In my haste, I forgot that retina display is just a marketing term rather than a specific resolution. But 4K is still higher than what's available for retina at the moment.
As screen sizes goes higher, I think 4K is plenty of detail for home cinema. 8K would probably be best suited for the commercial cinema screens. I doubt most home owners would be able to discern anything higher than 4K. Even with the retina display on the 15" MBP, it takes a moment to notice the difference from a regular screen. And that's for a static display! With moving images, it's even harder to tell the difference. Extreme high-res would probably be fantastic for digital art galleries I suppose. Can you guys remember when plasma display panels were first launched and they were used to demo virtual fish tanks? Many people thought that the fish were real and those panels were not even in 1080p yet. And no doubt 4K movies would require a substantially higher storage capacity than blu-ray that needs to be commercially viable.