We already have 2560x1600 ~10" tablet panels coming out soon so we will see 4K in computers monitors as well. TVs are far away, which actually makes 4K less useful in them. Viewing distance is an important part when figuring out "retina" resolution. iPhone 4 is retina because human eye can see approximately 300 pixels per inch when viewed from one feet, which is the typical phone viewing distance. Bring it closer and you can see the individual pixels. Or at further away, you need less pixels to make it "retina". 150DPI is "retina" from two feet, a regular viewing distance for computers. TVs are usually viewed around 8 feet away, making 1080p 55" TV already "retina".
30" 4K display would be ideal with its 147DPI. The extra pixels would actually be useful. In TVs, you need a big TV, most likely a projector, to benefit from 4K - or you must sit very close to your TV.