Second Moment of a Random Sum via the Tower Property

Let $N \sim \operatorname{Poisson}(4)$ and, given $N = n$, let $S = X_1 + \cdots + X_n$ where $X_i \stackrel{\text{iid}}{\sim} \operatorname{Uniform}(0,1)$. Use the tower property and the identity $E[S^2 \mid N] = \operatorname{Var}(S \mid N) + (E[S \mid N])^2$ to find $E[S^2]$.