I'm on a 37.5 flex time, can do work from 8 to 6 but can start 10 and leave 4 each day. While I stick to a routine of 9 to 5.30 ad leave early on Friday. The 4 days week makes sense for some industries but with mine I just don't think we can. It's not about how productive we can be it's about what needs to be done and when. Onsales, websites and whatever need changing and making during the week, even at weekends. I would say if we had less hours and only 4 days we'd feel far more rushed, which isn't a good thing.
However, I will note that those that work from home it makes more sense. I choose to work in the office, better environment, heated, good computers etc but when I was forced to work from home during lockdown I found myself working 9-5.30 every day with very little breaks, it felt like I was working more with little reward, so IF people actually do this at their work from home, as long as they get their work done, why not a 4 day week?