I worked at a startup that did a four day work week. It was awesome for the most part. The problem is, they would rely heavily on the four day work week to be able to pay less in salary, which is completely fair and very worth it to a lot of people. The issue came in when they pretty much explicitly told people they were expected to work on Fridays anyway, defeating the benefit of the four day work week, but allowing them to keep salaries lower and put "four day work week" on job listings.

They've threatened to get rid of it multiple times, claiming it's still a trial period (over a year of having it), which is fine, but they don't mention that it's a trial at all to new hires. I know a lot of the company that would leave in a heart beat if they got rid of it since it was a such a big part of why they chose the job.

I left, got a more interesting job that paid a lot more and they kept trying to get my to stay by referencing the unlimited PTO they gave me but I didn't use (less than a week taken in two years) and the four day work week that I hadn't taken advantage of since I joined. Needless to say, I got a big raise and I work the same amount that I did before.

An extra day off is absolutely additional compensation and is worth a significant amount. It's just that in my (small sample size and limited) experience, it's used as a crutch and there were pretty much expectations that you work on that extra day, negating the time off. I hope most other four day work week companies don't do this and I just had a bad draw, but that's my anecdotal experience.