Can devices even handle this resolution? Display wise, not right now, but Snapdragon 800 devices have enough power to natively encode 4K, which is staggering in itself. The imminent Xperia Z Ultra for one, along with re-jigged S4 and a bunch of others towards end of year.
My back of napkin maths shows that 4K resolution would have to be displayed on a 7″ or smaller device to have such a high pixel density. That’s way beyond what the human eye can discern, and I think it’s safe to say that’s plenty for anybody. Of course, it may never eventuate if OEM’s can’t see a business case for it.