4. Let $H$ be the orthocenter of $\triangle A B C$, and $O$ be the circumcenter of $\triangle A B C$. If $|\overrightarrow{H A}+\overrightarrow{H B}+\overrightarrow{H C}|=2$, then $|\overrightarrow{O A}+\overrightarrow{O B}+\overrightarrow{O C}|=$ $\qquad$ .