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You could take validating from the original 5 day week (instead of the then norm of 6 day week) which Henry Ford introduced about 100 years ago.

I don't follow. If validation of the 5 day week implies the validation of the 4 day week, then... well, I am sure you see where this is going.

Well the logical conclusion is a 0 day week which is obviously not going to work. There must be some sort of balance somewhere.

Not really. The logical conclusion is that productivity becomes high enough to just use fewer employees.