Let $$A = \frac{1}{8} \left(\begin{array}{llll} 9 & 1 & 3 & 3 \\ 1 & 9 & 3 & 3 \\ 3 & 3 & 9 & 1 \\ 3 & 3 & 1 & 9 \end{array}\right).$$ Show that $A \in \mathcal{S}_{4}(\mathbb{R}) \cap \mathrm{Sp}_{4}(\mathbb{R})$.