functional equations - How to find a such function $f:3mathbb{N}+1to 4mathbb{N}+1$

How to find a bijective function $f: 3\mathbb{N}+1\to 4\mathbb{N}+1$ such that

$$f(xy)=f(x)f(y),\forall x,y\in 3N+1$$

If i let $x,y\in 3\mathbb{N}+1$ then there exists $n,m\in \mathbb{N}$ such that $x=3n+1,y=3m+1$

but I have no idea how I can find a such $f$, Is there a method please ?