As regards the increased productivity argument I would add the ECON 101 dimension: if the 4-day workweek were as productive as it's made out to be successful firms would have introduced it already and the fuddy-duddies would have gone to the wall.

Having taught ECON-101, I'll also point out that (1) this argument assumes a complete network of perfectly competitive markets which (2) is nothing like what we have.

Firms are not productivity optimizers even in the traditional marginalist theory, they are profit optimizers, and there are many reasons why there can be productivity increases that are not reflected in increased profit for labor hour. Indeed, the massive departures we have to make from the real world in order to arrive at a model in which profit optimizing and productivity optimizing are equivalent should themselves signal that in general, they are anything but equivalent out in the real world.