4. As shown in Figure $1, A B$ is the diameter of semicircle $O$, quadrilaterals $C D M N$ and $D E F G$ are both squares, where $C, D, E$ are on $A B$, and $F, N$ are on the semicircle. If $A B=10$, then the sum of the areas of square $C D M N$ and square $D E F G$ is $\qquad$

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4.25 .

As shown in Figure 4, connect $O N, O F$.
Let $C N=x, E F=y$, then
$$
\begin{array}{l}
x^{2}+(x+D O)^{2}=25, \\
y^{2}+(y-D O)^{2}=25 .
\end{array}
$$
(1) - (2), we get
$$
\left(x^{2}-y^{2}\right)+D O \cdot(x+y)=0
$$

Since $x+y>0$, therefore, $D O=y-x$. Hence $x^{2}+y^{2}=25$.