I needed lower gearing than the 14-20t and 50/47t on my early-50's E. Christophe, and as the original chainset was fully amenable to use with a narrow, modern chain, I cobbled up an Ultra-5 freewheel based on a (slightly modified) Shimano UG Z-series 5s freewheel.

This gave me five speeds within the travel range of the original Simplex 4s Tour de France derailer.

The body modification as I recall involved a slight shortening of the large-form cog interface on the outer body, where the largest three cogs spline onto. I did this on an as-needed basis (to get the 2-3 position spacer to move inward), no precision lathe cutting but just using a grinder and Dremel.
I probably could have more-easily just expanded one of the steel, small-form spacers from a Dura-Ace 7s freewheel over the full large-form spline, but which would have left a large open gap in the spacer, which wouldn't really have been an issue.
Note that the "2-3" spacer I refer to is the one between the second and third smallest cogs.

I got the TDF derailer to play quite nice with my bigger, 14-24t "U-5" freewheel. The UG cog teeth really help with smooth shifting (keeping the shifting response decently "early" even using the flexible modern 7s chain).

Note that many 4s freewheels to be found these days will have French threading.