I had to move to 4days week due to covid. Must say that, unexpectedly, it was very good experience. It felt almost like 50/50 work/free days. I was more focused at work. I was more productive. Although, honestly, I don't think I was able to deliver same output as 5days week. The way I see it, you have only a few hours per day where your output is at the highest level. When you cut off 1 day you also decrease that hours. You can't just squeeze them into other days, because it doesn't work like that.

I actually learnt two lessons:

4 days a week is something I'm looking forward when I have a profitable side project.

5 days x 6 hours imo is better than 5 x 8 hours, better to be less tired and maximize next day's most productive hours than to be tired and burn out eventually