smc

Creating equations, we get $4\cdot \frac{1}{2\pi r}=2r$. Simplifying, we get $\frac{1}{\pi r}=r$. Multiplying each side by $r$, we get $\frac{1}{\pi }=r^2$. Because the formula of the area of a circle is $\pi r^2$, we multiply each side by $\pi$ to get $1=\pi r^2$. Therefore, our answer is $\boxed{1}$