The formula for the arc length of a subtending angle is the following:
[tex]s=2\cdot\pi\cdot r\cdot\frac{\alpha}{360\~}[/tex]
according to this, apply the information into the formula
[tex]s=2\cdot\pi\cdot4\cdot\frac{72}{360}[/tex]
simplify the result
[tex]\begin{gathered} s=\frac{576}{360}\pi \\ s=\frac{8}{5}\pi \end{gathered}[/tex]
