Answer
${$40}$ billion
Work Step by Step
Recall that for Okun's Law, every one-percentage point increase in unemployment above its natural rate causes a 2% negative GDP gap.
So, in this scenario, the unemployment rate is 4% above the natural rate, which correlates to a 8% GDP gap, and because the GDP is ${$500}$ billion, then the output being forgone to cyclical unemployment is $0.08\times 500={$40}$ billion.