This is interesting to me because I see a lot of need for better work/life balance and people trying many different ways to achieve that. But I think personally the reason you'd have a hard time finding a company willing to take that compromise (lower salary, fewer days per week) is because it's just you which makes you kind of stand out a bit compared to everyone else there. It also opens a can of worms for the employer who will have to make case-by-case decisions about every other person in the company who sees you getting 4 days and 4/5 the pay, and suddenly all run to ask HR for a similar deal. It's just going to be a big logistics headache for them.

My career was mostly a 4 day 32 hour a week job with the US government. For them it was a way to spread limited resources across many different efforts. You might look for a 32 hour apportionment with the feds. Perhaps there is some way to take advantage of limited resources in a private enterprise. I don't know how you'd go about it though.