The 4% Rule (also called the Trinity Study rule) suggests you can withdraw 4% of your portfolio in your first year of retirement, then adjust for inflation each year, with a high probability of not running out of money over 30 years. It is a useful starting point, but it has real limitations. The original study used US-only data from 1926-1995, assumed a 30-year retirement (too short for most FIRE retirees), and did not account for taxes, healthcare costs, or international diversification. Modern research by Wade Pfau, Karsten Jeske (ERN), and others suggests that a safe withdrawal rate may be closer to 3.25-3.5% for 50+ year retirements. The FI Plan uses Monte Carlo simulation rather than a single historical backtest, stress-testing your specific plan across 10,000 scenarios with varying return sequences.