A 314 in my 4CV has a 10 diff/ 20 shaft, but this might vary, either between years of 4cv and later Dauphine with 314
Early 4cv had a 289, later ( as two cars overlap) have 314
Dauphine early is 289, later 314, early Dauphine Gordini 318, later 325 or 330....not sure without looking in books and being an internet expert , which I’m not.
4cv shafts should be same length as common mod was wacking Dauphine Gordini arse end straight in 4cv, cutting inner wings to get spring mounts in, I may be able to get you some 4cv shafts, but can’t for a couple of weeks cos geezer has been opened up for some nasties chopping out, so he’s laid up, and I wouldn’t promise anything I’m not certain I can come up with.