Can we at least agree that 40 hours / 5 days a week is an arbitrary amount of work? Not so long ago the norm in many countries was 6 days. So why do you think the current amount is the optimal?
I think if we pursue this policy the newly available days need to be normalized so that the day is set aside for personal work.
Personally I'd like to see us slowly ease out of employment and demand that more people work for themselves. By most proposals of a four day week are simply against working.