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 largeforehead.
That woman must be housing a huge brain... 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 largeforehead that you can fit all five fingers.
That guy grows his hair into his eyes so you can't see his gigantic fivehead.