4K seems to be the minimum resolution needed to get a sharp image in the headset. This is because when you view a 360 image it gets stretched across a virtual sphere instead of just across the screen. That stretching seems to be about a factor of 2 times – probably a little more – than the pixels on the screen. Because of that, you need 2 times the resolution in video, which is what 4K is. You are right that higher resolutions are better, but to send the minimum amount of data with good resolution, 4K seems to be the right balance.