I started working a 4 day work week a few years ago and would not go back. The benefits to the employee are obvious, but consider those for the company: it gets a motivated, energised employee, and if like me they're a parent or for some other reason the employee has appointments (school, Dr etc), the employee often schedules them during the day off (or moves the day off to accomodate them), meaning the employee isn't taking any time off (usually, paid!) for them - I suspect I've had a lot less PTO than fulltime colleagues.