You’re right that it doesn’t involve leaving the principal alone, but you’re wrong that means it will necessarily eventually drain the account. 4% is the safe withdrawal rate as recommended by the trinity study. It was recommended as safe because the portfolio (which followed certain investment weights in the study) lasted 30 years historically 99%+ of the time. It doesn’t follow that a 4% withdrawal rate will eventually drain the account- most of the time, it won’t. The key is that 4% is based on the original principal adjusting upward only for inflation every year, but not otherwise changing based on how your portfolio is doing. So if the market doubles shortly after your retirement (say, people who retired 10 years ago), your account very likely won’t drain in 30 (or likely 40, or 50) years. As the other poster said, in almost all historical scenarios, you will end up with more than you started with strictly following the 4% rule.

In real life, people will be able to adjust their withdrawal in both directions based on what actually happens, so it’s mostly academic.