4. If $x_{0}$ is a root of the quadratic equation $a x^{2}+b x+c=0(a \neq 0)$, set
$$
M=\left(2 a x_{0}+b\right)^{2}, N=b^{2}-4 a c .
$$

Then the relationship between $M$ and $N$ is ( ).
(A) $M<N$
(B) $M=N$
(C) $M>N$
(D) $N<M<2 N$
4.B.
$x_{0}=\frac{-b \pm \sqrt{\Delta}}{2 a}$. Then $\left(2 a x_{0}+b\right)^{2}=b^{2}-4 a c$.