4. As shown in Figure 1, the diagonals $A C$ and $B D$ of quadrilateral $A B C D$ intersect at point $O$. $S_{\triangle A O B}=4, S_{\triangle C O D}=9$. Then the minimum value of $S_{\text {quadrilateral } A B C D}$ is ( ).
(A) 22
(B) 25
(C) 28
(D) 32
4. B.

As shown in Figure 1, let $S_{\triangle M O D}=x, S_{\triangle B O C}=y$. Then $S_{\text {quadrilateral } A B C D}=4+9+x+y \geqslant 13+2 \sqrt{x y}$. From $\frac{x}{9}=\frac{4}{y}$, we have $x y=36$. Therefore, $S_{\text {quadrilateral } A B C D} \geqslant 13+2 \sqrt{x y}=13+12=25$. Hence, the minimum value of $S_{\text {quadrilateral } A B C D}$ is 25. At this point, $A B \parallel D C$, meaning quadrilateral $A B C D$ is a trapezoid.