Perm Question?

I want get a perm and people say use pink and grey rods. How many inches does my hair need to be. Iknow you need to wrap it around the rod but iwant to make sure my hair is long enough before i go spend the money. What i am trying to achieve is distinct curls about pinky size. My hair is about 4" inches long. I have been wanting a perm for many years and i think i am ready so i want it to be right