The lens is just too small for 4K. It is impossible for that many different photons to pass through it at the same time. Photons are in practice about one millionth of a meter big, so about 1000 could fit beside each other passing through a 1 mm lens. But this is only valid for wide angle pictures, fish eye optics. Real phone cameras use just a limited angle, a limited view of this, removing even more photons. Even 1080p is often more than the cameras can really do.
IAARP, I Am A Real Physicst