This advert is the earliest mention, that I am aware of, of the Boizot 4 speed derailleur. I believe that, like the Boizot 5 speed system, the 4 speed system used a chainwheel that moved sideways to provide a straighter chainline.
The text appears to suggest that Boizot can offer a choice of 2, 3, 4, 5 or 6 speed gear systems set up with the développements you desire (in increments of 0.20 metres). That is is quite an offer.
(Source gallica.bnf.fr/ Bibliothèque nationale de France)