There's a happy medium imo. Output / value vs hours has diminishing returns over time and such: - 4 days != 80% output (it's more like 85%) - 3 days != 60% output (it's more like 70%)

I may be biased though as I run https://4dayweek.io/ - Software Jobs with a better work life / balance.

I wrote about the advantages of a 4 day week (for employers) here:

https://4dayweek.io/hire-engineers