The real reason for unlimited PTO to to skirt a very burdensome labor law in California. In California, you must be paid your final paycheck on the day you end employment. For tech workers, that could be a lot of cash a company suddenly has to come up with since PTO would have to be paid out on top of normal salary. I know it's such a problem that smaller companies with accrued PTO reject high paid CA candidates even if they have rights to work there.

As for 4 day work weeks. That's always a special topic for me because while it's great for certain kinds of workers, it's just materially bad for a lot of others. And then there's the elephant in the room that shows that literally every school district that moved to 4 day weeks has worse outcomes for children. Their grades are lower and health outcomes are worse because schools act as a safe harbor for underprivileged children.

I do know plenty of companies that play with schedules, but I've seen people tend towards 5 8 hour days. The only thing I've seen have success is flex hours, vacation minimums (which is really just accrued under a different name), and forced time off usually in December or in the summer if have a natural workaholic staff.