4. (8 points) Selling a product with a profit margin of $25 \%$, if you want to increase the profit margin to $40 \%$, then the selling price should be increased by $\qquad$ $\%$.


【Answer】Solution: $1+25\% = 125\%$
$$
\begin{array}{l}
1+40\% = 140\% \\
\quad(140\% - 125\%) \div 125\% \\
=15\% \div 125\% \\
=12\%
\end{array}
$$

Answer: The selling price should be increased by $12\%$.
Therefore, the answer is: 12.