Does the 4% rule assume I spend the same amount every single year?

Yes, and that rigidity is its biggest limitation. The classic rule takes your first-year withdrawal and increases only for inflation, ignoring how the market performs, which is why it needs a large buffer to survive worst-case sequences. Real retirees rarely behave that mechanically.

More flexible strategies improve outcomes and often allow a higher average spend. Guardrails methods, popularized by Guyton and Klinger, cut spending modestly after bad years and raise it after good ones. A simple approach is to trim discretionary spending in the years following a market drop. Because most people naturally spend less when markets fall, the fixed-spending assumption tends to be pessimistic. If you are willing to stay flexible, you can typically start a bit higher than 4% while keeping your ruin risk low.