$E:\frac{x^2}{16}+\frac{y^2}{9}=1$. For three distinct $P,Q,Q'$ on $E$, let $M(P,Q)$ be midpoint of $PQ$, $M(P,Q')$ midpoint of $PQ'$. Max distance between $M(P,Q)$ and $M(P,Q')$ is