4. (8 points) Integer division equation: $a \div b=c \cdots r$, if $a$ and $b$ are both multiplied by 3, then ( )
A. $r$ remains unchanged
B. $r$ is multiplied by 3
C. Both $c$ and $r$ are multiplied by 3
D. $r$ is multiplied by 3
【Solution】According to the problem, we have $a \div b=c \cdots r$ which can be rewritten as $a=b c+r$. If we multiply both sides of the equation by 3, we get
$$
3 a=3(b c+r)=3 b c+3 r
$$

Rewriting it in the form of a remainder gives us
$$
3 a \div 3 b=c \cdots 3 r
$$

Therefore, the correct choice is: $D$.