To answer your first point, I'm assuming 4 days a week presents itself as a good compromise between worker happiness and productivity given our level of technol

To answer your first point, I'm assuming 4 days a week presents itself as a good compromise between worker happiness and productivity given our level of technology and efficiency. Two or three days of work a week would be better for happiness, but our society isn't productive enough to reasonably manage it. As for whether our society as a whole would be as productive, I would reckon that the increased consumption (especially on leisure), and the increased amount of innovation due to decreased supply of labour and free time for individuals would offset the 20% reduction in labour hours.