4″ is pretty thick for a typical asphalt driveway. Here in AZ most roads, drives and parking lots are about 2.5″ like you said.
As long as he uses and compacts his GSA (Grade Stabaliztion Aggragate) everything should turn out fine. 3/4 mix will give you fairly rough looking finish, 3/8 will give a smoother looking finish and so on down the line.