The conversation about the 4% rule deserves more nuance than most posts give it.

William Bengen's original research showed that a 4% annual withdrawal rate would have sustained a 30-year retirement through every historical period in the data. That's the basis for the rule.

For people retiring at 65: the 30-year horizon roughly matches. For people retiring at 45: the 4% rule needs stress-testing over a 50-year horizon. The research on 50-year periods is less definitive; many researchers suggest 3.0-3.5% is safer for very early retirees.

The other variable the simple rule ignores: spending flexibility. Retirees who can reduce spending modestly in down years (cutting travel, dining out, etc.) survive market downturns far better than those with fixed expenses equal to their withdrawal rate. A flexible spending approach allows a higher starting withdrawal rate with lower sequence-of-returns risk.

I use 3.5% as my planning target, which implies a slightly larger FIRE number, but also more resilience to scenarios the historical data didn't include.