Euler Phi Function of 4

Example of Use of Euler $\phi$ Function

 * $\map \phi 4 = 2$

where $\phi$ denotes the Euler $\phi$ function.

Proof
From Euler Phi Function of Prime Power: Corollary:
 * $\map \phi {2^k} = 2^{k - 1}$

Thus:
 * $\map \phi 4 = \map \phi {2^2} = 2^1 = 2$

They can be enumerated as:
 * $1, 3$