4. Given two planes $\alpha, \beta, \gamma$, the angle between each pair of planes is $\theta$, and $\alpha \cap \beta=a, \beta \cap \gamma=b, \gamma \cap \alpha=c$.

If proposition 甲: $\theta>\frac{\pi}{3}$; proposition 乙: $a, b, c$ intersect at one point. Then
(A) 甲 is a sufficient condition for 乙 but not necessary
(B) 甲 is a necessary condition for 乙 but not sufficient
(C) 甲 is a necessary and sufficient condition for 乙
(D) None of (A), (B), (C) is correct
4.【Analysis and Solution】Let $a$, $b$, and $c$ intersect at one point. From a point inside the trihedral angle, draw three rays perpendicular to $\alpha$, $\beta$, and $\gamma$, respectively. The angle between each pair of rays is $\pi-\varphi$, where $\varphi$ is the plane angle of the dihedral angles that are equal in the trihedral angle. The total sum is 3 $(\pi-\varphi)\frac{1}{3} \pi$. When $\varphi>\frac{2}{3} \pi$, if $a$, $b$, and $c$ do not intersect at one point, they are parallel, in which case $\varphi=\frac{1}{3} \pi$. Answer (A).