6. (8 points) As shown in the figure, two circles with a radius of 4 cm are placed as shown, and the two shaded areas in the rectangle are equal. Points $A$ and $B$ are the centers of the two circles. Therefore, the length of $AB$ is $\qquad$ cm. $(\pi$ is taken as 3 )



The translation is provided as requested, maintaining the original formatting and structure.
$\frac{1}{4}$ of the circle's area is $\frac{1}{4} \pi 4^{2}=12$.
The area of $\frac{1}{4}$ of the circle is equal to the area of a small rectangle, and the area of two rectangles is 24 square centimeters.
$$
24 \div 4=6 \text {. }
$$

Therefore, the answer is: 6
【Solution】Solution: According to the problem, we know: