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.

Personally I don’t find an ethics violation. If you’re meeting your output expectations who cares how many hours you work. I certainly don’t as a manager at a large public company. I fully expect most ppl are getting some amount of personal stuff done during the day, or power napping between meetings. Just don’t be a dick and make the rest of the team pick up your slack.

Oh, I think it's expected that we get some amount of personal stuff done during the day, and don't feel the need to hide this from my coworkers or even boss. At the lower end, there are few "white collar" desk jobs where you can't, say, take a phone call from a family member during the work day, or take coffee breaks whenever you want, or have to hide any of this from your boss.

If it got to the point where I felt like I had to hide the amount of time I was spending "working"/"not working" from my boss, then even if it were ethical (I still think it depends on the situation), I would just find it too stressful and unpleasant an experience, to be routinely hiding or lying about what I did with my day.

To me, doing 'some amount' of personal stuff during the day (which should be acceptable at any job), is still an entirely different thing than "I don't work Fridays, don't expect to see me online". Qualitatively, not just quantitatively.

"meeting output expectations" is tricky, I think, because what is the amount of "output" expected from "a person week"? It obviously varies for different people, at different times in their career, different skills, and just from week to week different things going on their lives/stress level. Knowing that I am not "cheating" my week is part of what makes me feel justified and honest in pushing back if a workplace does try to assign more work than is possible. Barring emergencies (and "your failure to plan is not my emergency") -- we should not be expected to work extra hours to get more work done. We get done in a week what we get done in a week, and then we come back the next week and do more -- to some extent, as engineers, the amount we can get done in a week is simply defined by what we do get done in a week.

But still, my real point is that even using this humane "you get done what you get done" approach, in our line of work, most organizations would find, I predict, that people working only 4 days a week (80% time), get done 90% or more of what people working five days a week do.

What is 4-10s?

If a typical workweek is (8 hr) * (5 days) = 40hrs, I think gp is referring to (10 hr) * (4 days) = 40hrs

Four 10-hour workdays, as opposed to five 8 hour days.