Also a lot of jobs have enough flex hours and vacation that it's close enough to 4 days/week anyway.

The downside of shorter workweeks is it may cause a loss of productivity at the endpoints of the week. For example, there is often a marked slowdown on the Thursdays and Mondays before and after 3-day weekends. So you are not just losing an extra day but more like 1.5 days due to slowdowns and loss of productivity.