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.
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On Wed, Jun 9, 2021 at 3:38 PM Warner Losh <imp@bsdimp.com> wrote:

> So I've seen a number of places that talk about Unix TS 3.0 -> 4.0 -> 5.0
> progression and how System III was released and System V was released and
> System IV was internal only.
>
> What I've not seen is the "why" part of this. Why was it internal only?
>
> Warner
>
>