I don't think this is true. First, as others have pointed out, you can absolutely do a full day of manual labour. I've done this myself before. The hardest day of my life was unloading 12 kitchens from the back of lorries and moving them across a muddy building site. I woke up at 6:00, got home at 18:00 and slept the instant I hit the sheets.
Secondly, we do work for more than 4 hours a day. In fact, I work in my sleep. The brain doesn't just shut off. The reason you can't just "keep going" at the keyboard for more than 4 hours is your hands are faster than your brain. But the brain doesn't stop. It keeps crunching away. Every morning I find myself with new insights and new ideas that I didn't have the day before. I'm not just being paid for the hours I spend bashing away at the keyboard, I'm being paid to engage my brain with the business.
Secondly, we do work for more than 4 hours a day. In fact, I work in my sleep. The brain doesn't just shut off. The reason you can't just "keep going" at the keyboard for more than 4 hours is your hands are faster than your brain. But the brain doesn't stop. It keeps crunching away. Every morning I find myself with new insights and new ideas that I didn't have the day before. I'm not just being paid for the hours I spend bashing away at the keyboard, I'm being paid to engage my brain with the business.