For any vertex $x\in G$ denote by $N(x)$ the set of its neighbours. Let $a\in G$ be a vertex of maximal degree $\deg a$, then take $b\in G\setminus N(a)$ and $c\in N(b)$. If $c\in N(a)$, it means there must exist $d\in N(a)$ so that $d\not \in N(c)$, otherwise $\deg c = \deg a + 1$, contradicting the maximality of $a$. Then $abcd$ is such a seating. If $c\not \in N(a)$, again it means there must exist $d\in N(a)$ so that $d\not \in N(c)$, otherwise $\deg c = \deg a + 1$, contradicting the maximality of $a$. Then $abcd$ is such a seating.