There was never a System IV and System III was purely marketing, look at the docs it's PWB 3.0 but the few AT&T marketing/branding folks around didn't want to use the term 'Programmer' when it tried to compete with IBM (which was Charlie Brown - AT&T CEO - dream in those days).  As I understand it, the issue was that said folks wanted to sell to commercial sites (and in particular the 3B20S), and naming the software to drive with a term like 'programmer' would be a turn-off for less technical (traditional big blue) customers.

As for why PWB 4.0 never got released I believe that was there was a lag between what Summit (NJ) did as the 'UNIX Support Group' and what was being released as 'product' (NC).  Remember this is all part of the post-Judge Green world @ AT&T and things were not really smooth.  >>>my impression<< (and like you would love to know more), was that the release of PWB 3.0 was part of the pre-consent decree SW release rules, and by the time of System V, it was full steam ahead.  PWB 4.0 was finished up during that funny period between the two.