I've been looking closely at the Unix V4 tape and it looks awfully similar to Unix V5. The dates are from June 1974 and it includes various tools such as diff which are in the Unix V5 manual but not in the V4 manual. I suspect that this tape was made shortly before the Unix V5 release which according to https://en.wikipedia.org/wiki/Research_Unix was also released in June 1974 whereas Unix V4 was from November 1973. I'm not convinced that we really have much more than we did before December.