Basically every study shows a four day week works best. The issue is why we never go with what the study shows.
Naive question but if it works best wouldn't companies that have a four day work week outperform theirs peers and because of that grow faster, and become more common? I see the opposite in most startups that have a 6 day work week to get ahead of the "slowly moving" 5 day work week competition.
Problem is there's no such thing, monopoly powers, government subsidies, inter-company issues, contracts.
All these things can mean that a less functional, more wasteful and less productive organisation performs (in the sense of the metric that companies care about , line go up) better than a 4 day week startup.