While the forehead is defined as the part of your face above the eyebrows to your hairline, a fivehead is ascribed to someone that has a rather large forehead.
That woman must be housing a hugebrain... Did you see the size of her fivehead?
A "four-head" is when you can only fit four fingers between your eyebrows and your hairline. You have a fivehead if you have such an abnormally large forehead that you can fit all five fingers.
That guy grows his hair into his eyes so you can't see his gigantic fivehead.