Disagree. I used to think this. Especially if you consider that the average employee is at work for 8 hours and probably only is productive for, maybe, 4. There's stats on this if you Google it.

Then I started working for myself and building my discipline. I can easily have 4-6 hour work sessions without trying.

The other week I worked for 12 hours straight whilst doing intermittent fasting. I took a few breaks to stretch, exercise and whatnot for health reasons. So although I disagree with the premise of your post, I do agree that you should probably move around every now and then to prevent getting in a rut. And especially stop trying to work as soon as you get distracted because then you'll just keep sitting and waste away time. And yes, 4 hours can be more productive than 12 hours if you're getting distracted.

But 8 hour productive hours is still more productive than 4 productive hours.

I think it's about finding ways to decompress and keep yourself on high and energy and focus. And keep experimenting with it until you find your flow. E.g. repeating certain songs, being in a certain environment, doing a certain task, drinking a certain beverage.

And of course, get enough sleep. Then again, it also depends on your goals and how much you enjoy it. Side note, I'm a workaholic and I enjoy what I do. That also matters. I also have developed a strong work ethic over the last 7 years.