4. Let $A=\{(x, y) \mid 0 \leqslant x \leqslant 2,0 \leqslant y \leqslant 2\}$, $B=\{(x, y) \mid x \leqslant 10, y \geqslant 2, y \leqslant x-4\}$ be point sets on the Cartesian plane $x O y$. Then $C=$ $\left\{\left.\left(\frac{x_{1}+x_{2}}{2}, \frac{y_{1}+y_{2}}{2}\right) \right\rvert\,\left(x_{1}, y_{1}\right) \in A,\left(x_{2}, y_{2}\right)\right.$ $\in B\}$ is the area of the figure formed by ( ).
(A) 6
(B) 6.5
(C) $2 \pi$
(D) 7
4. (D).

Set $A$ is
square $O A B C$,
Set $B$ is
Rt $\triangle D E F$, as
shown in Figure 1.
The midpoint of $O D$
is $M(3,1)$,
The midpoint of $A E$ is
$N(6,1)$, the midpoint of $B F$ is $P(6,4)$, the midpoint of $C F$ is $Q(5,4)$, and the midpoint of $C D$ is $R(3,2)$.
The area of the formed figure is
$S_{\text {MNTQR }}=S_{\text {MNITF }}+S_{\text {DPQR }}=7$.