Sorry for the (super) late reply.
It might simply be due to the (lack of) quality of a given camera's sensor.
When considering image quality, the amount light delivered to each photosite on the camera's sensor will largely determine the image's purity.
A larger sensor which is the same resolution as a smaller sensor will have larger photosites and therefore collect more light at each point, leading to a less noisy image.
Similarly, adding a larger lens which collects more light and competently delivers it to the camera's sensor can improve image quality for a camera although it still has the same sized sensor that it did before the large lens was added.
Back on topic, though, no I haven't had the opportunity to shoot any 4K video.
I'd be interested in hearing from anyone who has, though.