Behold System V; what about systems I-IV?

System V (by AT&T) is the ancestral Unix, and I have used it. But I have never read or heard anything about systems I-IV, other than they existed once upon a time in the dim past. How effective were they, and were they widespread? Is it possible to get one's hand on the code and/or install it? Does anyone here have any experience with these earlier systems?