Exercise 4. Four circles $\mathrm{C}_{1}, \mathrm{C}_{2}, \mathrm{C}_{3}, \mathrm{C}_{4}$ of identical radius $\mathrm{r}$ are internally tangent to a circle of radius $R$. We set $C_{5}=C_{1}$. We assume that for all $i=1,2,3,4$, the circles $C_{i}$ and $C_{i+1}$ are tangent. Determine the value of the ratio $\frac{r}{R}$.