A decade ago I was working 5 days a week from an open-plan office, and mentally clocked out around 2pm each day. Office culture meant I was just moistening a chair from then until it was acceptable to leave around 6pm.
For the last few years I've been working 35-45 hour weeks from home, especially during COVID, and that was fine productivity-speaking.
I'm currently working 4 days from a co-working space, and 1 day from home. I know my productivity drops when I'm at home, but I'm not sure why this changed in the last year or so.
I think a 4-day week would probably work well for me at the moment. But I'm not sure - I could end up with Tuesdays looking a lot like Mondays do at the moment. Or the extra day to do all the other stuff would be amazing and I'd really focus on the other four days. shrug
Developer productivity is a strange beast. Can't measure it. Can't control it directly. I know it when I see it, but I couldn't explain why or how.