IME, you are correct. I would not be in favor of a 4-day work week if it was still 40 hours a week. Ten hours is too much.

We used to have "summer flex time" in our office. You could have any schedule you wanted for the summer, as long as you submitted it to the boss at the beginning of the summer, so others knew when they could expect you to be in. One summer, I went for a four-day work week, and regretted it. Ten hours a day was very dreary and exhausting.

The next summer, I went with alternate Fridays off, which was better. But most people ended up taking alternate Friday afternoons off, because even nine days a week resulted in work days that were too long.

I suppose it would be different for different types of jobs. At least in WA and AK, it's common for low-skilled industrial workers to work sixty and sometimes seasonal eighty-hour weeks. You get paid overtime for anything above forty.The added overtime would make these jobs more appealing.