In model theory, the spectrum of a first-order sentence $\phi$ is the set of all positive integers $n$ such that $\phi$ has a model of cardinality $n$. Consider the sentence $\phi$ in a signature containing a single unary function symbol $f$: $\phi = \forall x (f(f(f(x))) = x \land f(x) \neq x)$ Which of the following describes the spectrum of $\phi$?