4) As shown in Figure 2, in the shape composed of seven small squares, line 1 divides the original shape into two parts of equal area, the intersection point of 1 with $AB$ is $E$, and the intersection point of 1 with $CD$ is $F$. If the sum of the lengths of segments $CF$ and $AE$ is 91 cm, then the side length of the small square is $\qquad$ cm.
4) $a=26$ .