There have been some nice 360 cameras, but like you say, for dashcam use you need at least a 100 megapixel sensor, and even then it is impossible to mount it in a position where the view in some directions isn't blocked by peoples heads, and then lens technology for a single 100 megapixel fisheye lens where you are only really interested in the bit around the edge of the lens where the quality is worst is not sufficient quality yet. If you want 360 then you are much better off with multiple conventional lenses positioned where they have a good view and then combine the results into the 360 image, then having obtained the 360 video, you find that the original multiple recordings are better quality and give better evidence.
4 would be nice, front - back - left - right, and the option of a 5th IR camera specifically for the inside would be even better, then you don't need to choose between inside exposure or outside exposure for the side views.
Adding a 4th camera to the A139 seems like a small step forwards, hopefully the next generation of processors will have no problem supporting it, and with the thin coax cables it is much easier to install and get tidy than older dual remote cameras.