4. Let the side length of the equilateral $\triangle ABC$ be $a$, and $M, N$ be the midpoints of $AB, AC$, respectively. $D$ is any point on $MN$, and the extensions of $BD, CD$ intersect $AC, AB$ at points $E, F$ (as shown in the figure). Then the value of $\frac{1}{CE}+\frac{1}{BF}$ is
4. $\frac{3}{a}$