Four years is a peculiar length of time.
Four years is not a particularly brief period of time. It’s roughly five percent of your entire life, give or take a percentage point, if you’re living in the first world. It’s not especially long, either. If you told someone you did four years in prison, they might be piqued but hardly horrified; if you only got to spend four years with the truest love of your life, you’d be left dramatically unfulfilled.
Forty-eight months seldom marks an epoch, but it’s more than enough time for noticeable evolution. Perhaps this is why most high schools and undergraduate programmes are four years long.