[dcb]

I think you can just use 4 controllers and not worry about a differential, conceptually anyway. The dolphin uses two controllers instead of a differential.

But hub motors are heavy (unsprung weight=bad) and some gearing is usually called for, so sprockets and chain are a hard-to-beat-for-efficiency option, and is trivial and inexpensive to implement on the non-steering wheels, and provide super-easy ratio changes, plus redundant powertrains for uber reliability (especially if you have a pack for each controller like the dolphin).

So it is very cost effective and simple and flexable and potentially very reliable to just drive the rear two wheels with two motors. It isn't clear to me that the "dream" of 4wd is all that universal.