\[f(x) = \sqrt{x - 4} + \sqrt{4 - x},\ \]
\[\left\{ \begin{matrix} x - 4 \geq 0\ \ \\ - x + 4 \geq 0 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ }\left\{ \begin{matrix} x \geq 4 \\ x \leq 4 \\ \end{matrix} \right.\ \text{\ \ \ }\]
\[D(f) = \left\{ 4 \right\}.\]