(4) $l$ and $l^{\prime}$ are mutually perpendicular skew lines, $l$ is parallel to plane $\pi$, and $l^{\prime}$ lies in plane $\pi$. The locus of points in plane $\pi$ that are equidistant from $l$ and $l^{\prime}$ is ( ).
(A) Line
(B) Ellipse
(C) Parabola
(D) Hyperbola
(4) $\mathrm{D}$ Hint: In the plane $\pi$, take $l^{\prime}$ as the $x$-axis, and the projection of $l$ in the plane $\pi$ (the intersection line of the plane through $l$ perpendicular to $\pi$ and $\pi$) as the $y$-axis to establish a rectangular coordinate system. In the plane $\pi$, the distance from a moving point $M(x, y)$ to $l^{\prime}$ is $|y|$, and the distance to $l$ is $\sqrt{x^{2}+a^{2}}$. Thus, $|y| = \sqrt{x^{2}+a^{2}}, y^{2}-x^{2}=a^{2}$ (where $a$ is the distance between the skew lines $l$ and $l^{\prime}$). Therefore, the required locus is a hyperbola.