4. If $x=123456789 \times 123456786, y=$ $123456788 \times 123456787$, then the size relationship between $x$ and $y$ is ( ).
(A) $x=y$
(B) $x<y$
(C) $x>y$
(D) Uncertain
4.B.

Let $a=123456$ 789, then
$$
\begin{array}{l}
x=a(a-3)=a^{2}-3 a, \\
y=(a-1)(a-2)=a^{2}-3 a+2=x+2 .
\end{array}
$$

Therefore, $x<y$.