Ok, I can believe the authors point but why 4 years specifically? I wonder what he feels the difference between say 2yrs and 4yrs at the same company is, becau

Ok, I can believe the authors point but why 4 years specifically? I wonder what he feels the difference between say 2yrs and 4yrs at the same company is, because it definitely seems possible to see and experience the things he's talking about in less than 4yrs.