4. As shown in Figure 1, in trapezoid $A B C D$, $A D / / B C, A D$ $=2, A C=4, B C=6, B D=8$. Then the area of trapezoid $A B C D$ is ( ).
(A) $4 \sqrt{15}$
(B) 16
(C) $8 \sqrt{15}$
(D) 32
4. A.

As shown in Figure 6, draw $DE // AC$ intersecting the extension of $BC$ at point $E$. Then the area of $\triangle AC$ is equal to the area of $\triangle BDE$ while $BD=BE=8, DE=4$. Therefore, $S_{\triangle ADEE}=4\sqrt{15}$.