20 cm. Flatten the surface of the rod into a rectangle measuring 4 x 12 cm. With the string being wound four times around the rod, it covers 3 cm of the rod’s length on each turn. Recalling the Pythagorean theorem from your high-school geometry class, \(\sqrt{3^2+4^2}=\sqrt{9+16}=\sqrt{25}=5\). Multiply that by 4 and you get 20 cm.