(4) Given unit vectors $\vec{a}=\left(\frac{5}{13}, \frac{12}{13}\right), \vec{b}=\left(\frac{4}{5}, \frac{3}{5}\right)$ on a plane, which of the following relations is correct? ( ).
(A) $\vec{a} \perp \vec{b}$
(B) $(\vec{a}+\vec{b}) \perp(\vec{a}-\vec{b})$
(C) $(\vec{a}+\vec{b}) / /(\vec{a}-\vec{b})$
(D) $\vec{a} \perp(\vec{a}+\vec{b})$
(4) B Hint: Since $\vec{a}$ and $\vec{b}$ are both non-zero unit vectors, then with $\vec{a}$ and $\vec{b}$ as sides, $\vec{a}-\vec{b}$ and $\vec{a}+\vec{b}$ as diagonals, a rhombus is formed, so $(\vec{a}+\vec{b}) \perp(\vec{a}-\vec{b})$.