Ok to use pin 5 and yes in the firmware it should be a 5 not a 4 as according to the spec sheet you are quoting which came from the reprap wiki is ground. The fun part in All of this is you need a 5v power source and a ground which is all well and good they are right together but pin 5 is on the other side of the rail. So you will need another connector to set that pin up as your servo pwm.