4. $a$, $b$, $c$ represent the three sides of a triangle, the nature of the roots of the equation $b^{2} x^{2} + \left(b^{2}+c^{2}-a^{2}\right) x+c^{2}=0$ is ( ).
(A) No real roots
(B) Two equal roots
(C) Two unequal roots
(D) Cannot be determined
4. (A).
$$
\begin{array}{c}
\Delta=\left(b^{2}+c^{2}-a^{2}\right)^{2}-4 b^{2} c^{2}=4 b^{2} c^{2} . \\
{\left[\left(\frac{b^{2}+c^{2}-a^{2}}{2 b c}\right)^{2}-1\right]=4 b^{2} c^{2}\left(\cos ^{2} A-1\right)<0 .}
\end{array}
$$