If the salaries are not affected, then this is extremely unprofitable for employer. Paying for a 4 day work similar to 5 day work is disastrous, as you pay more for less job done.
As to my job, I remember we had a working day 8:30 to 5 with a 30min lunch break, later it has changed into 9 to 6 with 1 hour lunch break. Same 8 hour working day, but it is more profitable for employer. Previously a lot of people were late for job and 30 min lunch break last almost for an hour, as it is impossible to have a lunch in such a short time in our surrounding. Now people rarely came late for job and instead of 1 hour lunch spend around 45min.
I find 8 hour working day 5 times per week perfect timetable. If I work more, similar to you, I wont have time for a proper rest and do my own things.
That's the reason it's not applicable for every sector; it could be useful in technology, IT departments, accounting, or marketing services. Certainly, it cannot be applied to every instance, but a workaround can be implemented to still keep your business open for 5 days while adopting the 4 work days a week plan by simply distributing your workers' day off. Some will work from Monday to Thursday and others from Tuesday to Friday. Honestly, while it sounds ideal, I believe that it won't be widely embraced by employers, and it's understandable for the reasons you mentioned earlier. Personally, I'd be happy if I even had weekends off while working 8 hours per day, 5 days a week, and 20 to 24 days of paid time off (PTO).
Honestly I am not like this, I do not believe in this situation. If you like your job, which I do, then you can work 24 hours straight and you would be happy about it. However if you hate it, even 1 hour is terrible. I have never been a good student, I hated school, but as soon as I started working, I took more and more whenever I can, I did the jobs and tasks that others didn't want to, I worked as many hours as I possibly can, life sometimes makes sure that you can't, but whenever I can, I tried to work as hard as possible.
This is why I believe that I have been in a good job, and if I am required to work 24 hours, sometimes I do that, and I am very happy to do it. I get paid for it anyway, not like I am tricked into working, I work hard, but I am very well rewarded for it too.
If you're an entrepreneur and it's your own business, it's understandable to work your ass off, especially at the very start of your business. No matter how much you enjoy your job, you should never neglect your personal time.