  We show that every indecomposable conical symplectic hypersurface of
dimension four is isomorphic to the known one, namely, the Slodowy slice $X_n$
which is transversal to the nilpotent orbit of Jordan type $[2n-2, 1, 1]$ in
the nilpotent cone of $\mathfrak{sp}_{2n}$ for some $n\ge 2$. In the appendix
written by Yoshinori Namikawa, conical symplectic varieties of dimension two
are classified by using contact Fano orbifolds.
