I have no idea.
Only skimmed the thread so sorry if this has been said,
but I have a hard time believing that one can only grow 4" of hair per year even with the best of retention, which makes me think that rate has something to do with retention or health and not your natural maximum growth rate.
I really don't think anyone can grow 3" of hair per month, I'm sorry but that seems totally ridiculous.
This hater's going to hate.