Or you work 8h less and gain 24 of "contiguous hours that you control" per week.

Yes, the disproportionately huge increase in contiguous time off is the big win in my book.

We could just work Monday and Tuesday with a 40 hour marathon each week fueled by coffee, donuts and an occasional pizza.

Well... :D You can't add the gained time twice. It is like saying, instead of spending 20 USD on coffee, you spend 15 USD. You didn't gain 10 bucks, you are still left with only five more than you would have otherwise.

Leisure hours in a 5-day work week: 72 (8 hours per day and 16 on weekends)

Work hours in a 5-day work week: 40

Difference: 32 hours

Leisure hours in a 4-day work week: 80

Work hours in a 4-day work week: 32

Difference: 48 hours

That’s the diffence. You had 72 hours before, now you have 80 hours of leisure time. It’s 8 hours more of leisure time.

GP - "another way to think about it is that it’s a difference of 16h"

You - "That’s the diffence."

In my reply I said the effective gain is 8 hours.

No matter what kind of math you do, working one day less doesn’t give you two extra days off.