4. Given $990 \times 991 \times 992 \times 993=\overline{966428 A 91 B 40}$, then $\overline{A B}=$
【Analysis】Since 99 | 990, we have $99 \mid \overline{966428 A 91 B 40}$, so 99 | $96+64+28+\overline{A 9}+\overline{1 B}+40 \rightarrow 99 \quad \mid \overline{A B}+247 \rightarrow \mathrm{AB}=50$