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Four years.
Four years. The math says you have spent the last 35,040 hours not doing the thing you used to do. The math also says that does not really mean anything anymore, because the thing you used to do has become the thing you used to do.
What changes between year three and year four
Not much, and that is the point. Year four is mostly continuation. The body, the mind, the social patterns have all stabilized. What remains is to keep being the person who does the thing you have been doing for four years.
There is no four-year-specific lesson that does not also apply to year three or year five. The lessons compound. The lessons do not change at integer year boundaries.