I did the exact same thing. It was amazing and well worth the 4/5ths pay. My mental health was improved and the only thing that made me eventually leave was that i was offered 3x the pay to work at a FAANG.

Startups trying to compete for talent should probably take note of the fact that it took 3x more pay for them to poach staff, not 20% more. I'm not saying you'll get away with paying 3x less but you also won't have to worry about losing staff if you're in the ballpark since the 4 day work week is that valuable to anyone lucky enough to have experienced it.