4. (6 points) There are three natural numbers $a, b, c$. It is known that $b$ divided by $a$ gives a quotient of 3 and a remainder of 3; $c$ divided by $a$ gives a quotient of 9 and a remainder of 11. Then, the remainder when $c$ is divided by $b$ is $\qquad$
【Solution】According to the problem, we have:
$$
\begin{array}{l}
b=3 a+3, \\
c=9 a+11 .
\end{array}
$$

Also, $c=9 a+9+2=3(3 a+3)+2=3 b+2$, which means $c \div b=3 \cdots 2$.

Therefore, when $c$ is divided by $b$, the remainder is 2. The answer is: 2 .