#### Answer

$s=2\pi$ units

#### Work Step by Step

Step 1: The formula to be used here is $s=r\theta$ where s is the length of the arc intercepted, r is the radius and $\theta$ is the central angle of measure in radians.
Step 2: $s=r\theta$
Step 3: $s=(4)(\frac{\pi}{2})$
Step 4: $s=2\pi$ units