I'm fully in favor of shorter work days regardless of how many days one works. However, when forced to pick between the two options, I strongly prefer working 4 full days, which I have been doing for more than a decade.
It could be just me, but I find the value of stopping around 3-4pm versus 5-6pm to be pretty limited. You still have this feeling that the main part of the day is almost over and you can't do any substantial personal tasks or serious relaxing before the dinner ritual.
Also, when you're the exception in your team to leave early, you're effectively "disappointing" your team every single day that you work. Where others might have 4PM meetings, you're never available. This invites a feeling of guilt, where you start to make exceptions, and incidentally stay longer. Others are learning that your 4-6pm unavailability is quite soft. Further, every single morning you're behind in email and chat, needing to go through what happened late in the previous day whilst others are already up to speed.
With a real day off (4 day workweek), you have a full day to do something really substantial. Time feels plentiful rather than scarce.
A full day off is a barrier that is much easier to defend. You're just not available at all on that day, and work and expectations easily adapt to that.
Overall, I think the problem with modern office work is that it's far too collaborative. We've normalized being in meetings for half of the day and having hundreds of emails and chat messages to wade through, whilst the actual work takes a back seat.
If you'd have a clean and clear work package, you could do the work in that package at your convenience in whatever way fits best in your life. But we don't have that. We start the morning fresh with an idea of goals for the day in mind, and at the end of the day we did almost none of it. Everything changed throughout the day. Too many dependencies, too many moving parts. That's the real problem.