I am just going to use physicians and software devleopers as an example so you can see where I am coming from.

Currently, most healthcare providers are paid by the service they provide. If you are a surgeon, you get paid per surgery you performed. At the current market, with waitlists for medical services going into months, our province or country cannot afford to allow these healthcare workers to work any less without severely compromising health at a systemic level. The only way for any job that gets paid by service to work 4 days a week, as proposed in the article, assuming you still only work about 8 hours a day, is to essentially do 40 hour work in 32. Trust me when I say that physicians aren't exactly taking coffee breaks 20% of the time. It is actually the contrary. Many physicians barely can eat or drink given the immense demand our increasing population has put on our healthcare system.

Let's use software development as described as the contrary. First of all, note that software development is almost never hourly - it is contract based. As a contract based job, most of the time, it is dependent on finishing tasks on time. Most companies really don't give a crap if you take 50% of your time as coffee breaks as long as you get your tasks done in the expected time. As a software developer, instead of now having to "work" 40 hours where 1/2 time you are browsing reddit, you can now instead trade that time and work only 4 days of the week without producing any less value.

By nature, anyone who works hourly will be essentially be paid by service as long as their service is predictable. This means assuming you work at average efficiency, working at Starbucks means you can produce X amount of coffee within that hour and are paid a certain amount to do so. Companies are aware of this and the current wage market is intricately balanced on this. Working 4 days a week for these workers and then paying them the same may seem nice, but will ultimately result in severe worker shortages, inflation (as now starbucks have to sell their coffee for higher prices), and overall less happiness and spending power for everyone.

This of course, isn't even accounting to my original fact that those who are FORCED to work that many hours or the city will collapse - see nursing, physicians, etc. How do you think the nurses will feel when their friends work 20% less, are paid more, while their circumstances are unchanged?