$6,000 / 0.04 = $150,000. in other words, if OP had saved $150,000 and was withdrawing 4% per year, they would have the same $6,000 as what the app is generati

$6,000 / 0.04 = $150,000. in other words, if OP had saved $150,000 and was withdrawing 4% per year, they would have the same $6,000 as what the app is generating. the analogy is imperfect, because presumably the OP is spending some time to generate that app income; it's not totally passive.