If you are saying while including one and the #, you get 6. If you are saying withOUT including on and the #, then it is 20 (from what I came up with)!
4 = 4, and 4 = 2 x 2, so either a single prime number to the power 4-1 (23, 33, etc.), or two different prime numbers, each to the power 2-1, for example, 2x3, 2x5, 3x5, etc.