This is awesome. How did you manage to convince your employers, of 4 day work week? I was just thinking about it, I don't think there is any chance that I can convince my own employer of it :(
I managed to convince my last five employers to let me work a four day week or less. Usually, I asked for it during the interview and accepted the job offer where they said "yes". That will rule out some jobs, but overall I can recommend the approach, as long as you don't have trouble finding jobs even without that requirement.
I read the employee handbook, in its entirety. It said that in order to get all fringe benefits, a salaried employee must work a minimum of 30 hours a week. Another section said that if full time salaried employees worked less than 40 hours a week, then their paycheck would be prorated.
I did give up 20% of my salary and occasionally when there was an emergency and everyone showed up Saturday morning to make a delivery, I would also show up and not bill for those hours. This only happened a few times a year.
If you don’t ask for stuff, you are unlikely to get it.