The 40 hour / 5 day work week has long been dead: we need the four day week to try to get back to 40.
Implicit in the 5 day / 40 hour work week was that for the most part a husband would work for an employer during that time while a wife would do 40+ hours of unpaid labour at home. So you really have an 80+ hour work week for a husband and wife team.
When the labour at the employer was factory-like shift work, a 40 hour week was truly a 40 hour week. (Domestic labour had no such boundaries.) But as work became more office oriented, many employers and ambitious employees shunned the “9 to 5” in favorite of longer hours. As more work became digital, computers came home and that time became work time. The 40 hour work week morphed into the 45, 50, 60 even 70 hour work week.
As a result of all this, men and women find themselves with very little free time for themselves. If they aren’t working, they are taking care of their family, nuclear and extended. Or they are taking care of their homes. Most people would tell you their responsibilities+work is not 8 hours a day: it’s like more like 12 or 14. Plus time on the weekend.
While employers might feel this benefits them, some of your potentially better employees — mainly women at this point — are lost to the overwhelming responsibilities they have to deal with. And even if they are somehow managing, they are likely unhappy and likely to burnout and quit, and a cost to the employer as much as the employee.
Given all this, a 4 day work week would still likely result in a 40+ work week, but at least the resulting weekend would give people a chance to truly rest. Nowadays, Saturday is a day to catch up on home responsibilities and Sunday is a day of either religion of recreation**. We need an extra day. At least.
Too many employers are still focused on things like hours worked (or worse, working in the office). Employers need to know what their purpose is and tie their employees effort to that, while recognizing employees are not machines or cogs but people. And not just employers: we need that incorporated into our society as a whole.
For more on this topic, you can easily do a search and find many pieces on how it is beneficial. To start, read this in the pro-business Wall Street Journal: The Four-Day Workweek Gets Shorter With Practice – WSJ
(** At my employer there would be many people who would be sending out and responding to emails up until noon on Saturday, breaking off to take care of their family, their homes and themselves, and then start sending out and responding to emails on Sunday evening. Their weekend was from noon on Saturday to Sunday at 6 pm.)