4. Let $z_{1}, z_{2}$ be complex numbers, and $\left|z_{1}\right|=3,\left|z_{2}\right|=5,\left|z_{1}+z_{2}\right|=7$, then the value of $\arg \left(\frac{z_{2}}{z_{1}}\right)^{3}$ is . $\qquad$
4. Let $z_{1}, z_{2}, z_{1}+z_{2}$ correspond to points $Z_{1}, Z_{2}, Z_{3}$ in the complex plane, as shown in the figure, then
$$
\cos \angle O Z_{2} Z_{3}=\frac{3^{2}+5^{2}-7^{2}}{2 \times 3 \times 5}=-\frac{1}{2},
$$

so $\angle \mathrm{OZ}_{2} Z_{3}=\frac{2}{3} \pi$.
Thus,
$$
\begin{aligned}
\arg \left(\frac{z_{2}}{z_{1}}\right) & =\angle Z_{2} O Z_{1} \\
& =\pi-\angle O Z_{2} Z_{1} \\
& =\frac{\pi}{3},
\end{aligned}
$$

or $\quad \arg \frac{z_{2}}{z_{1}}=\frac{5}{3} \pi$.
Therefore, $\quad \arg \left(\frac{z_{2}}{z_{1}}\right)^{3}=\pi$.