4. In quadrilateral $A B C D$, $\overrightarrow{A B}=\vec{a}+2 \vec{b}, \overrightarrow{B C}=-4 \vec{a}-\vec{b}, \overrightarrow{C D}=-5 \vec{a}-3 \vec{b}$, the shape of quadrilateral $A B C D$ is ( ).
A. Rectangle
B. Parallelogram
C. Rhombus
D. Trapezoid
4. D. $\overrightarrow{A D}=\overrightarrow{A B}+\overrightarrow{B C}+\overrightarrow{C D}=-8 \vec{a}-2 \vec{b}=2 \overrightarrow{B C}$.