4. As

shown, hexagon
$A B C D E F$
is composed of
five squares,
with the side
length of each
square being
1 cm. A line through $A$ intersects $E D, C D$ at $M, N$ respectively. If the areas on both sides of line $M N$ in this hexagon are equal, then the length of $E M$ is: $\qquad$ cm.
4. $\frac{\sqrt{5}-1}{2}$