8. As shown in Figure 4, in parallelogram $A B C D$, $A B=2, B C=$ $2 \sqrt{3}, A C=4$, a line $E F \perp A C$ is drawn through the midpoint $O$ of $A C$, intersecting $A D$ at $E$ and $B C$ at $F$. Then $E F=$ $\qquad$
8. $\frac{4 \sqrt{3}}{3}$