I'm pretty sure the "4" comes from mapping the letter grades A, B, C, D, F. "A" has the most points (4) and "F" the least (0). Most of the time -- but not always -- the letter grades have already been mapped from a numerical scale or percentage, e.g., A=[90-100], B=[80-89], C=[70-79], D=[varies?], F=[can't fudge a D].