Building in the hardware to shoot 4K could be great- a high value benefit that would give us something to watch as we slowly transition to 4K TV sets. Right now, the lowest cost 4K camcorder is still a couple thousand, so if anyone could get it into a phone that costs $600-$1000, that would be a lot of value right there.
Building a phone screen to display all of the pixels in a 4K video is practically pointless. Your eyes couldn't see that level of detail on such a tiny screen. So let the screen max out just beyond what the best set of human eyes can see. But I would be interested in shooting at 4K (even though that will beg for big storage on board).