Ive found that there are days when I can’t do more than 2 hours without struggling, and some days where 10 hours go by without too much effort. Like you, I probably average 4 hours a day, but I don’t want to artificially limit it if I’m in a groove.

At my old team - in a fairly well known non-Faang software company - we planned sprints assuming 4 productive hours a day, so the 4 hour figure is probably not too extraordinary.