[sbaildon]

Anecdotally from myself, colleagues, friends, blogs, and random forum posters, 4 solid hours of knowledge work is right on the nose.

I work on my own time, starting almost immediately after waking up with the sunrise. I get in 3-4 hours while the world is quiet leading up to lunch time; then I'll cook some lunch and go training. Post-lunch work is autopilot admin or operation tasks. There are days where focus is amped up to 11 and the brain fog doesn't set in—but those days are rare.

I'd say that your feelings of "cheating the system" are ingrained by a culture that doesn't apply to your profession.