Circle $C_1$ and $C_2$ each have radius $1$, and the distance between their centers is $\frac{1}{2}$. Circle $C_3$ is the largest circle internally tangent to both $C_1$ and $C_2$. Circle $C_4$ is internally tangent to both $C_1$ and $C_2$ and externally tangent to $C_3$. What is the radius of $C_4$? $\textbf{(A) } \frac{1}{14} \qquad$ $ \textbf{(B) } \frac{1}{12} \qquad$ $ \textbf{(C) } \frac{1}{10} \qquad$ $ \textbf{(D) } \frac{3}{28} \qquad$ $ \textbf{(E) } \frac{1}{9}$