What I think this short article misses are a lot of the questions that are being asked here.
The 4-day work week isn't 4 x 10, it's more like 4 x 9. The pay is the same as 40 hours.
It's not a "everyone is out of the office on Friday" schedule. Some people are out on Friday (lucky dogs), and some on Monday/Tuesday/Wednesday/Thursday. You still have, to the customer perspective, people in the office every day of the week.
Further research also reveals that this 4-day work week isn't for all employees, just most. So, if you need to work 5 days a week, you apparently can.
My view on it, is that this could also lend itself to making the 4-day office work week, and 1-day working from home; for those who are "guilty" about not being in the office that 5th day.