I have managed to do this at a succession of several jobs now. When I briefly tried to go back to 5 days, it was really hard. It seems insane to me now that anyone works 5 days a week. (And I do know plenty of people work even more, and I am very lucky).
Fridays are for doctor's appointments and car repair and grocery shopping and errands. Which probably does mean I'm away from "work hours" for these things less than I would be full-time, I just schedule such things for Fridays where possible. In general, I am pretty convinced that I'm at least 90% as productive on 80% time as I would be on 100%, maybe more.
Working at non-profit academic, I'm definitely not making my "market" salary. I could be making a lot more than I am now, even at 80% of a salary, at various more typical organizations. But I'm not sure how viable the 4-day week is with potential employers -- even if I'm willing to take a paycut to 80% of salary, which I am! Unlike the OP which is giving everyone a 4-day week at full salary? My impression is that most high-paying software engineering jobs/organizations are not going to let you do 4-days a week? Which is crazy when they could get me for 80% of the cost with most of the productivity -- but I guess the high-paying organizations aren't exactly trying to save money on salary.
I really don't think I'd go back to 40-hour-a-week-standard unless I had to.