4. As shown in Figure 1, in $\triangle A B C$, $C D$ and $C E$ are the altitude and median on side $A B$, respectively, with $C E=B E=1$. The perpendicular bisector of $C E$ passes through point $B$ and intersects $A C$ at point $F$. Then $C D+B F=$ $\qquad$
4. $\frac{7 \sqrt{3}}{6}$