Majority of my team works 4 days/week, but do so by working 4x9 hours (36 total) instead of 5x8 hours (40 total). We don't really do less work (or take longer for projects).
Some observations:
* The folks working 4 days are a bit less likely to pick-up 'side activities' (e.g. help organise an external event, lead some of the recruiting efforts, etc.), because a day away takes out 25% of active time in one go.
* Ratio overhead vs work is probably better for the part of the team working 4x9 vs 5x8. It turns out they can miss many of these meetings and still be fine without additional catch-ups. (unfortunately not in my power to just remove it completely)
* I believe in people having broad responsibilities ("deliver this project in the best way you see fit") rather than "work X hours". We don't downscope projects because someone works 4 vs 5 days, but no one has ever remarked on it either. This type of responsibility sometimes means people wrap-up early, sometimes they feel responsible enough to pick something really important up in the evening/weekend and take a few hours off on another day.
* I think the ~10% difference wouldn't be measurable for software (or AI in our case) development anyway, we're not a call center where you can measure 'minutes called'.
Final notes:
* This is my European/Dutch perspective. I understand this trend is stronger in the nordics/Germany/Netherlands than in the southern countries. Having managed a team in the US in the past, I find it hard to imagine how this would work in the US culture I experienced.
* I have no desire to own souls ;)