To clarify to a since flagged (by someone else) comment:
When i say "I'm at least 90% as productive" -- I mean I am more productive per hour worked. Not that I end up working 90% hours but only getting paid for 80%.
I really do work 80% hours, only four days a week, Monday to Thursday. I am not expected to and don't work on Fridays, Saturdays, or Sundays.
I just get done I think 90% or more of what I would get done in five days, in four days. As seems to be commonly reported by people in this situation.
Some people have also said "Yeah, but in my remote software dev job, I work even less than 80% of my expected hours, I just don't tell them, and I get paid a full salary."
I'm not gonna say much about the ethics of that, I guess I think it depends on the specific situation. But regardless, I'd myself just be stressed out if I had to be less than honest about my work. And my 80% schedule is clear -- I really don't work on Friday, nobody expects to be able to reach me on Friday (any better than they could on a Sunday or at 4am, unless I'm on-call), I don't check my email or look at my work calendar, I am really not working. And that's the whole benefit to me -- it's a different thing than using lots of hours for personal social media or something (which, sure, I do to some extent too whether 4 or 5 days a week.