The idea being that you are more productive and more focused because you are more rested.

It's probably tnaked to a decrease in stupid preventable mistakes.

Most companies aren't able to count the costs of stupid preventable mistakes. 20% is the number I've seen, but it's low because of hard to measure factors.

The companies that do understand the costs of stupid preventable mistakes usually go in a "work life balance" binge to improve their own bottom line.

No clue. I got that number from articles about multiple pilot runs. Well, honestly, it depends; on the company structure and what they do. A minority of companies made a bit less. But on average in the range 10% to 20% more. Guesses are, because not much happens at friday anyway and employees are less tired on monday. 4 days workweek just seems to hit a better balance.