4. (6 points) Define the operation: $A \triangle B=2A+B$, given that $(3 \triangle 2) \triangle x=20, x=$
【Solution】Solve: $(3 \triangle 2) \triangle x=20$,
$$
\begin{aligned}
(2 \times 3+2) \triangle x & =20, \\
8 \triangle x & =20,
\end{aligned}
$$
$$
\begin{aligned}
2 \times 8+x & =20, \\
16+x & =20, \\
x= & 20-16, \\
x & =4 ;
\end{aligned}
$$

Therefore, the answer is: 4 .