4. In trapezoid $A B C D$, $A D / / B C, E, F$ are points on $A B, C D$ respectively, $E F / / B C$ and $E F$ bisects the area of the trapezoid. If $A D=a, B C=b$, then the length of $E F$ is $\qquad$
4. $\sqrt{\frac{a^{2}+b^{2}}{2}}$