4. The vertex of the parabola is at the origin, the axis of symmetry is the $x$-axis, and the focus is on the line $3 x-4 y=12$. Then the equation of the parabola is $(\quad)$.
(A) $y^{2}=-12 x$
(B) $y^{2}=12 x$
(C) $y^{2}=-16 x$
(D) $y^{2}=16 x$
4. D.

From the vertex at the origin and the axis of symmetry being the $x$-axis, we know the equation of the parabola is $y^{2}=2 p x$. In $3 x-4 y=12$, setting $y=0$, we get the focus at $(4,0)$, so $p=8$.