I can tell you from my experience as a freelance, about more than 15 years ago. I actually did something even cooler: 3 day-weeks (12h/day) -- which is higher than 4d work week.

It was beyond awesome!

Unfortunately, it didn't last for more than roughly 2 years, since I made the insanely stupid decision to run the Romanian office of an international outsourcing company (all went downhill from there).

The plan is to eventually return to this, but in the current environment, it's pretty hard to do.