I've been noticing this type of thinking a lot on HN. It simply isn't true. I am not in the tech industry (if you search my comment history you'll find I do underground tunneling) but everybody I have interacted with at my firm that is above the junior level (and the majority of those at the junior level) puts in far more than 4 hours of concentrated, high-quality thinking a day.
Out of the normal 45 to 60 hours I work a week when I am doing design work, at least 75% of that time is spent doing high level thinking. I am not an anomaly in my firm.
I don't accept nor do I respect the idea that a person is only good for a couple of hours of high quality work to their employer per day.