4. (8 points) As shown in the figure, semi-circular lace is attached around a square with a side length of 10 decimeters. The number of circular paper pieces needed is ( ).
A. 8
B. 40
C. 60
D. 80
【Solution】Solution: 10 decimeters $=100$ centimeters,
$$
\begin{array}{l}
100 \div 5 \times 4 \div 2 \\
=20 \times 4 \div 2 \\
=40 \text { (pieces) }
\end{array}
$$

Answer: You need to buy 40 circular paper pieces.
Therefore, the correct choice is: $B$.