As an employee it felt great because 3-day weekends were awesome when I was young and single. However, my coworkers with kids and families had mixed feelings about it because it was difficult to coordinate with daycare, school, and other activities.
As a manager, 4-day workweeks felt like a minefield. It turns out that a lot of people can't really produce more than a few hours of work per day. A lot of people also struggle to get going on Mondays and tend to leave early on the last day of the week, so in some cases people became significantly less productive even if they were at the office (or online) for 10 whole hours each day. Some people treated it like a slippery slope, trying to casually move down to 4 x 8 hour days or less. Some people wanted to work 5-day weeks, which turns out to be difficult to mix with other people working 4-day weeks but longer hours.
I think 4-day weeks make sense for jobs that have a lot of dead time anyway. There are a lot of companies out there with way more staff than they need, so going to 4-day weeks doesn't change much. However, 5-day workweeks will still better for productivity, collaboration, and keeping people fresh each day. Unpopular opinion, I'm sure, but that's my experience.
5x8 by 4x10 (indeed I don’t imagine myself being able to produce code for 10 hours)
Well France is on 35h workweek anyway.
But we are advocating for a 4x7 = 28 workweek (likely with a paycut of course)
I feel this would be a net positive for a company.
Or it could be 5*5 = 25 workweek for that matter. Because when the task is complicated and creative, I don’t think I can work 7 hours most of the time
What I can think of is, that if you have deadlines and you mentally can't get away from tech-problems - just having a day that is free without something else to do might be not as fullfilling.