8. (10 points) Please fill in each arrow with an appropriate number, so that the number in the arrow indicates how many different numbers are in the direction the arrow points to. A bidirectional arrow indicates how many different numbers are in the entire direction of both sides of the arrow. Then, the four-digit number formed by the numbers filled in from left to right in the second row of the figure is $\qquad$ . (The figure is a $3 \times 3$ example)



保留了源文本的换行和格式，直接输出了翻译结果。
【Solution】Solution: According to the analysis, reverse deduce, start deducing from the first column. It is easy to get $M=1$, and the first column has three different numbers, so we get $N=3, O=2$;
$F$ points to the two numbers on the left, since $G$ points to the two numbers on the right which cannot be 3, hence $F=2$;
At $H$, $L$ can only be 1 or 2. If $H$ is 1, then $L$ would also be 1, and $B$ must be 1, which is clearly impossible because $A=1$, and $B$ points to the three numbers on the left, which already have 1 and 3, so $B$ can only be 2 or 3. Therefore, $H$ and $L$ can only be 2.

In summary, the numbers in the second row are already determined, which are: 3122.
The numbers filled in are as shown in the figure below:
Therefore, the four numbers that should be filled in the second row are: 3122.
The answer is: 3122.