IMO: I have never worked in a job where I would have been like "heck yeah, four day work week lets do it". Part of that is, there's kinda two distinct and separate camps, and sometimes its unclear which camp people are in and what we're fighting for. Is it "I work ~40 hours in four days" or "I work ~32 hours in four days"?
And my take is, I don't like either of these solutions.
I've never worked in a role where I feel like working 32 hours a week would deliver meaningfully similar results to working 40 hours. You're just gonna deliver less. That doesn't mean I'm coding for 40 hours a week, not even close, but there's so much admin and meetings (and even just finding availability for meetings!) that reducing net hours worked by 20% isn't going to have market success.
Increasing the hours worked on each day by 20%, but getting an extra day off, also sucks. There's more "things I gotta do every day" than "things I gotta do every week". I need every hour I've got most nights. Cooking, working out, reading, entertainment to keep the sad away, some types of shopping, these aren't by and large things that I can just say "lets wait and do them all on my extra Friday off!", they need to be done ~every day.
What you say is 100% my feeling as well: I am comfortable working 38-42 hours a week, and I want to by-and-large choose the hours and days I work to hit that. That is the best solution. If I need to schedule a dentist appointment for 1pm two weeks out, I want to do that without thinking even for a second that I need to check with "work", file PTO, etc. In exchange, I'll make up that hour by working until 6pm, or going to a coffee shop for a couple hours over the weekend, whatever feels like it makes the most sense for where I'm at and where the business is at. And, sure, there's core hours, there's meetings, we work around those; I'm talking policy, not the day to day.
This pattern of handling time off is so important to me that I have quit a job within the first month because they misrepresented how they handled PTO. I was told "oh yeah you can take off for a doctors, whatever bro no big deal", which turned into "oh no you've gotta file PTO, any time off throughout the day costs a full day and needs to be approved". I quit on the spot. Policies like that, four day work weeks, limited PTO, attract cogs, not high performers.