4. As shown in Figure $1, O$ is the center of a sphere with radius 1, points $A, B, C$ are on the surface of the sphere, $OA, OB, OC$ are pairwise perpendicular, $E, F$ are the midpoints of the arcs $\overparen{AB}, \overparen{AC}$, respectively. Then the spherical distance between points $E, F$ on the surface of the sphere is $\qquad$