4. (10 points) A school has two classes each in the third and fourth grades. Class 3-1 has 4 more students than Class 3-2, Class 4-1 has 5 fewer students than Class 4-2, and the third grade has 17 fewer students than the fourth grade. Therefore, Class 3-1 has fewer students than Class 4-2 by ____.
【Answer】Solution: $4+17=21$ (people)
$$
\begin{array}{l}
(21+5) \div 2 \\
= 26 \div 2 \\
= 13 \text { (people) } \\
13-4=9 \text { (people) }
\end{array}
$$

Answer: Class 1 of Grade 3 has 9 fewer people than Class 2 of Grade 4.
Therefore, the answer is: 9.