4. Given three quadratic equations in $x$
$$
\begin{array}{l}
a x^{2}+b x+c=0, \\
b x^{2}+c x+a=0, \\
c x^{2}+a x+b=0
\end{array}
$$

have exactly one common real root. Then the value of $\frac{a^{2}}{b c}+\frac{b^{2}}{c a}+\frac{c^{2}}{a b}$ is ( ).
(A) 0
(B) 1
(C) 2
(D) 3
4.D.

Let $x_{0}$ be a common real root of them. Then
$$
(a+b+c)\left(x_{0}^{2}+x_{0}+1\right)=0 \text {. }
$$

Since $x_{0}^{2}+x_{0}+1=\left(x_{0}+\frac{1}{2}\right)^{2}+\frac{3}{4}>0$, we have
$$
\begin{array}{l}
a+b+c=0 . \\
\text { Therefore, } \frac{a^{2}}{b c}+\frac{b^{2}}{a a}+\frac{c^{2}}{a b}=\frac{a^{3}+b^{3}+c^{3}}{a b c} \\
=\frac{a^{3}+b^{3}-(a+b)^{3}}{a b c}=\frac{-3 a b(a+b)}{a b c}=3 .
\end{array}
$$