4. The necessary and sufficient condition for the line $a x+b y+c=0(a, b, c \neq 0)$ to be symmetric to the line $p x+q y+m=0(p, q, m \neq 0)$ with respect to the $y$-axis is ( ).
(A) $\frac{b}{q}=\frac{c}{m}$
(B) $-\frac{a}{p}=\frac{b}{q}$
(C) $\frac{a}{p}=\frac{b}{q} \neq \frac{c}{m}$
(D) $-\frac{a}{p}=\frac{b}{q}=\frac{c}{m}$
4. (D).

The line symmetric to the line $a x+b y+c=0(a, b, c \neq 0)$ with respect to the $y$-axis is obtained by replacing $x$ with $-x$, resulting in $-a x+b y+c=0$. Note that the line $-a x+b y+c=0$ coincides with the line $p x+q y+m=0$ if and only if the necessary and sufficient condition is met.