4. In a cyclic quadrilateral $ABCD$, the lengths of the four sides in sequence are $AB=2, BC=7, CD=6, DA=9$. Then the area of the quadrilateral is $\qquad$
(2010, National Junior High School Mathematics League, Jiangxi Province Preliminary)
Because $7^{2}+6^{2}=2^{2}+9^{2}$, therefore, $BD$ is the diameter of the circumcircle of quadrilateral $ABCD$, i.e., $\triangle ABD$ and $\triangle CBD$ are right triangles. The calculated area of the required shape is 30.