For real numbers $a, b, c, d$, let
$$
\begin{aligned}
M= & \left(a^{2}+b^{2}-1\right)^{2}+\left(c^{2}+d^{2}-1\right)^{2} \\
& +2(a c+b d)^{2}, \\
N= & \left(a^{2}+c^{2}-1\right)^{2}+\left(b^{2}+d^{2}-1\right)^{2} \\
& +2(a b+c d)^{2} .
\end{aligned}
$$

Then the relationship between $M$ and $N$ is ( ).
(A) $M>N$
(B) $M=N$
(C) $M<N$
(D) Cannot be determined
4. B.
$$
\begin{aligned}
M= & \left(a^{4}+b^{4}+1-2 a^{2}-2 b^{2}+2 a^{2} b^{2}\right)+\left(c^{4}+d^{4}\right. \\
+ & \left.1-2 c^{2}-2 d^{2}+2 c^{2} d^{2}\right)+2\left(a^{2} c^{2}+b^{2} d^{2}\right. \\
+ & 2 a b c d) \\
= & \left(a^{4}+c^{4}+1-2 a^{2}-2 c^{2}+2 a^{2} c^{2}\right)+\left(b^{4}+d^{4}\right. \\
& \left.+1-2 b^{2}-2 d^{2}+2 b^{2} d^{2}\right)+2\left(a^{2} b^{2}+c^{2} d^{2}\right. \\
& \quad+2 a b c d) \\
= & N
\end{aligned}
$$

If special values are taken, although (A) and (C) can be excluded, (D) cannot be excluded.