Find all solutions of $\phi(n) = 4$ and prove there are no more. I have found that when n = 5, 8, 10, & 12 are the only solutions where $\phi(n) = 4$. I also know that I can spilt up the solutions to say that $\phi(n) * \phi(m) = 2 *2$. I am just not sure where to start with the proof.
Help would be appreciated.