Answer
$1.5$ radians
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: $6=(4)(\theta)$
Step 4: Multiplying both sides by $\frac{1}{4}$,
$6\times\frac{1}{4}=4\times\theta\times\frac{1}{4}$
Step 5: $\frac{6}{4}=\theta$
Step 6: $\theta=1.5$ radians