Proponents pretend like 4 day work weeks don’t reduce output with all sorts of dubious motivated reasoning.
A lot of it is “we can be more efficient” and therefore we don’t need to work as many hours, overlooking the obvious fact that if everyone can be 20% more efficient then logical thing to do is downside headcount by 20%, not keep 100% of the workforce at 80% of capacity.
It takes so long for a woman to go through the gestation period before giving birth. But when you get 9 women you can get 9 babies in the same period. But it still takes 9 months. I think the women just aren't working at full capacity for those 9 months. It should be possible to just get the 9 women to produce 1 baby in 1/9th of the period.
... Except the obvious argument for a 4 day week is that in a knowledge economy, downtime is more critical to meaningful work. People like Tolkien don't just write 9-5, there's no reason consultants or programmers should work that way, it just doesn't scale.