Is the 4% rule a guarantee that my money will never run out?

No. The 4% rule is a historical rule of thumb, not a promise. In the Trinity Study, a 4% inflation-adjusted withdrawal survived every historical 30-year window, but "survived" often meant ending with a large balance and occasionally meant cutting it close. It says nothing about periods longer than 30 years, and it assumes future markets look at least as good as U.S. history.

Researchers like Morningstar have argued that lower expected stock and bond returns may justify a starting rate closer to 3.3% to 3.7% today. The rule also ignores flexibility: real retirees adjust spending in bad years, which dramatically improves survival odds. Treat 4% as a planning anchor, not a set-and-forget autopilot, and stress-test your plan with the FIRE Calculator at wealthserene.com/tools/fire-calculator.