Quick answer: the classic 4% rule was calibrated for 30-year retirements ending in 1995. If your retirement could last 40-50 years — which planning to age 100 requires — a starting withdrawal rate of 3.0% to 3.5% is generally safer, and flexible guardrail strategies outperform fixed rates.
This guide explains where the 4% rule came from, why longer horizons change the math, and how to stress-test your own withdrawal rate with Monte Carlo simulation — free, no signup.