4. As shown in Figure $1, A B C D-A^{\prime} B^{\prime} C^{\prime} D^{\prime}$ is a cube. Any plane $\alpha$ is made perpendicular to the diagonal $A C^{\prime}$, such that $\alpha$ intersects each face of the cube. Let the area of the resulting cross-sectional polygon be $S$, and the perimeter be $l$. Then ( ).
(A) $S$ is a constant, $l$ is not a constant
(B) $S$ is not a constant, $l$ is a constant
(C) Both $S$ and $l$ are constants
(D) Neither $S$ nor $l$ are constants