I think 4:30 will be hard to market, as most people are stuck in the stone age. As you said, productivity is not a matter of hours, it's a mix of many things. But that's not what the majority believes. Many people still believe we work like ice-cream sellers, who need to be around, not matter what. My guess is most people want to pay only for the hours you work so it will be hard to explain why you want to be paid for 8 hours when you just work 4 hours (even with same productivity).

Personally I work every day. When I am tired and not productive, I work less. When I feel good, I work much. The minimum is usually 6 hours. Maximum is usually 12 -14 hours. At weekends, it happens it's just 2 hours a day.

I don't count in work time. I count in "fun". If I have fun, I automatically work more. If I am not having fun, I need to double-check if I am the right thing. If it's not, I go away. If it is temporary or something that needs to be done, I make sure I have some good relaxing part afterwards.