4. As shown in the figure, two national defense highways, each 40 m wide, intersect at an angle of $\alpha$. Then, the area of the common part (the shaded area in the figure) of the road surface at the intersection is ( ).
(A) $\frac{1600}{\sin \alpha}\left(\mathrm{m}^{2}\right)$
(B) $\frac{1600}{\cos \alpha}\left(\mathrm{m}^{2}\right)$
(C) $1600 \sin \alpha\left(\mathrm{m}^{2}\right)$
(D) $1600 \cos \alpha\left(\mathrm{m}^{2}\right)$
4. A