Second Moment of Centered Sum at a Poisson Horizon

Let $X_1,X_2,\dots$ be i.i.d. with mean $\mu$ and variance $3$. Let $N$ be independent of the increments and distributed as Poisson(4). Show that for the centered stopped sum $M_N=\sum_{i=1}^N (X_i-\mu)$, one has $E[M_N^2]$ equal to what value?