A historical stress test, not a law
Withdraw 4% of the starting balance, raise with inflation, and historical US portfolios survived 30 years in essentially every starting year tested. That is the entire claim. It says nothing about 45-year horizons, non-US markets, or your flexibility — which is why the honest planning range is 3.25–4.5% depending on exactly those factors.
The first five years are the fragile ones
Sequence risk concentrates early: retire into a bear market while withdrawing, and the shares sold never recover. Practical armor: 1–2 years of spending in cash, willingness to trim 10% after bad years, and delaying large one-time spending out of the first years. Retirees who survive the first five years at their rule almost always finish fine.
The upgrade path
Fixed rules are training wheels. Guardrail strategies (start ~5%, cut after losses, raise after gains), Social Security timing (delay to 70 for a bigger inflation-linked layer), and part-time income each add real safety. The 4% calculator here is the floor of the conversation, not the ceiling.