4. (3 points) Define new operations: $A * B=(A-B) \div 3, A \square B=(A+B) \times 3$, please calculate: (39*12) $\square 3$ = . $\qquad$
【Solution】Solve: $(39 * 12) \square 3$,
$$
\begin{array}{l}
=(39-12) \div 3 \square 3, \\
=27 \div 3 \square 3, \\
=9 \square 3, \\
=(9+3) \times 3, \\
=12 \times 3, \\
=36 ;
\end{array}
$$

Therefore, the answer is: 36.