Yeah, the H264 encoding on board will definitely be interesting but 4K data is a lot so I wonder how Companion can be modified to handle that. Either way still cool but may take some work 
If there is an easy way to implement 4K H264/265 on Companion, we would definitely be interested in making a USB module for it too!
Stacking and pixel binding are interesting approaches but my main concern is the amount of image noise from a smaller sensor due to tough underwater situations. Smartphones tend to deal with this by adding noise reduction and digital sharpening later on. Even with bright lighting, a smaller sensor will produce noise in the shadows which is why no smartphone sensor can’t beat an actual camera with a much larger sensor when both are recording at the same resolution. Interesting enough, most of the times, 1080P on an actual camera with a larger sensor will look way sharper and more detailed than 4K on a smartphone which I assume will be similar here. The image samples provided on their website don’t look too sharp to me but maybe those are just the initial revision