> Returns from a typical investment portfolio have been over 4% for the last 10-20 years, so that's the amount you can withdraw without depleting any principal.

Past results do not guarantee future performance. People in the FIRE community generally look at market performance since the final decades of the XIX century. For many, even these numbers do not guarantee anything, as the growth during these days reflected USA entering its golden age. Who knows if it will last through XXI century.