The famous French 35 hour workweek only applies to certain categories of workers though. And even then it’s only where overtime kicks in. Every French white collar worker I’ve known (probably about a dozen or so) was on salary and had similar working hours to me (in Canada).

Also, French productivity can be overstated in statistics. They have relatively high unemployment amongst youth and the low-skilled. This essentially removes the least productive from the stats, making it look higher.

Not to say that a 4 day week would be bad, but careful what you base it on.