4. (8 points) The figure below shows the trademark of a certain infant product, consisting of two hearts. Each heart is made up of a square and two semicircles. If the side lengths of the two squares are $40 \mathrm{~mm}, 20 \mathrm{~mm}$, then the area of the shaded region is $m m^{2}$. ( $\pi$ is taken as 3.14)
【Answer】Solution: As shown in the figure:
$$
\begin{aligned}
& \left(40^{2}+\pi \times 20^{2}\right)-\left(20^{2}+\pi \times 10^{2}\right) \\
= & 1600+400 \pi-400-100 \pi \\
= & 1200+300 \pi \\
= & 1200+300 \times 3.14 \\
= & 1200+942 \\
= & 2142 \text { (square millimeters). }
\end{aligned}
$$

Answer: The area of the shaded part is 2142 square millimeters. Therefore, the answer is: 2142.