There's a ratio that holds for arc lengths:
\(\dfrac{\text{arc length}}{\text{measure of angle subtended by arc}}=\dfrac{\text{circumference}}{\text{one full revolution}}\)
Denote the arc length by \(L\) and the angle subtended by it by \(\theta\). Then given a circle of radius \(r\), this relation says
\(\dfrac L\theta=\dfrac{2\pi r}{2\pi}=r\implies L=r\theta\)
So, the arc length in this case is simply
\(L=4\text{ cm}\times3=12\text{ cm}\)
