Purely answering the question... The pay, unless you bill by the hour. If you bill hourly and you can comfortably live on 80%, then you can aim for 4 days and flex if needed. It will give you time to work on side projects or just relax.
If you're salary, then the 4 day 'deal' is a Faustian bargain. You will be expected to be available for free on the fifth day, and you will rarely ever get it off. That's just the nature of business.
How do you expect that would happen? Does your company expect you to be available on Saturdays or at four in the morning? If not, why would they expect you to work during your other off times?
For me, switching to four eights was simply a matter of letting the right people know, then no longer showing up on Mondays. If you sent me an email on Monday, you'd get a reply on Tuesday. If you scheduled me into a meeting on Monday, I'd politely remind you of my schedule. It was never an issue.
Incidentally, I strongly recommend this schedule over five day weeks. The extra day makes way more difference to your quality of life than you would reasonably expect.
The chief difference is that on Saturday or 4am there aren't people sitting in an office wanting your "urgent" feedback on something they're working on, and possibly not even aware you're officially unavailable when they call your mobile. Obviously it applies more to people who are directly answerable to demanding clients or have unique abilities to fix/decide things than your average employee. And colleagues are definitely better at allocating things to other people when you're definitely away for a week or three.
I worked a four day week for several months due to accumulated holiday to use up, and seemed to field more direct sales enquiries on my Fridays off than the rest of the week. This wasn't entirely uncompensated, but it didn't feel like a three day weekend either.
If anyone wants to do this the key is:
1. Have some money in the bank so you're nor desperate for a job.
2. Have in-demand skills.
3. Get some practice negotiating.
4. Ideally do this at place you already work, or with consulting client: much easier than negotiating shorter workweek with people who don't know you.
(If anyone is interested I'm writing a book on how programmers can get to a sane workweek: reasonable hours, remote work, or even a shorter workweek, though that's harder. Initial email course is at https://codewithoutrules.com/saneworkweek/ until the book is done.)
And don't discount that people usually spend one of their two weekend days doing chores, business, and shopping. When you can push your personal business day from Saturday to Friday or Monday that gives you two full days off back to back.
4 days of work and 3 days of rest is, IMO, the perfect balance. You get enough time to rest that you actually look forward to going back to work (if you enjoy it).
Most of us do a lot of chores on the weekend that we can't do in the weekday because they're only open 9-5. A 3 day weekend would give us more time to ourselves.
However, I'd also take 50% and a 5-day week, if the team was guaranteed to be as pleasant to work on as the best team I've worked with.
I think in knowledge work - trading off time for salary is generally not a great move because we're almost always "on the clock."