In other laptop chargers the pin is of round shape, so that there is no orientation of direction for the connection i.e. always +pin of charger connected to +ve pin of laptop and -ve to -ve.
In Apple's version out of 5 pins, 1 pin (middle) is for the battery status which provides a signal to LED to change color red/green.
As the connecter is of rectangular type, suppose if there were only two pins (+ and -) then there would be wrong connection possibility depend on the direction of pin connection. So to ensure correct connection in both way 4 pins are needed.