The radius of $2^{\text {nd }}$ orbit of $\mathrm{He}^{+}$ of Bohr's model is $r_{1}$ and that of fourth orbit of $\mathrm{Be}^{3+}$ is represented as $r_{2}$. Now the ratio $\frac{r_{2}}{r_{1}}$ is $x: 1$. The value of $x$ is ___________.