I've had 4 day workweeks at two different times in my career. Once in my 20s, when I worked 10 hours days to get an extra day off. Didn't matter since I worked close to that anyway, and often more. The day off was flexible, so I chose Wednesday whenever possible. Two "weeks" of two days each worked great for me. Best schedule I ever had.

Another place, the salary was slightly lower (but not much due to the way they did levels), but the tradeoff was worth it. This was near the end of my time working for other people, so I was honestly on my way out anyway.

I think it's hard to make an entire company work on a 4 day schedule if you're interfacing with clients and so on. Or do the thing where half are off Monday, half off Friday.