>Every process, system, and best practice should be designed around helping people optimize for that 4 hours of really great work.

That's an accurate figure IMO. I have tried every which way to keep the quality and intensity up after the four-hour 'great work' mark, but I can't do it, and I think most people can't do it. There's some kind of daily mileage allowance we get, and most of us are running on fumes after 4 hours, it seems.