4. Given $z_{1}, z_{2} \in \mathbf{C}$, and $\left|z_{1}\right|=3,\left|z_{2}\right|=5,\left|z_{1}-z_{2}\right|=7$, then $\frac{z_{1}}{z_{2}}=$
4. $\frac{3}{5}\left(-\frac{1}{2} \pm \frac{\sqrt{3}}{2} \mathrm{i}\right)$ Hint: The angle between $z_{1}$ and $z_{2}$ is $\arccos \left(-\frac{1}{2}\right)=120^{\circ},\left|\frac{z_{1}}{z_{2}}\right|=\frac{3}{5}$