In addition, we're talking about a real withdrawal rate; a 4% real rate of withdrawal will be approximately a 6% nominal rate assuming inflation sticks around 2%. It's very unlikely you're going to maintain that from a balanced portfolio over the long term without depleting principle at all. Might be possible with an all-stock portfolio if you get lucky, but significant chance of failure if you get a poor sequence of returns.
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.
at this point a reasonable response is "there are no guarantees". If the 4% rule was back-tested through the great depression and generally came out fine, it's probably in the right ballpark.
As another FIRE blogger puts it, "3% or less is a near sure bet as anything in this life can be"
https://jlcollinsnh.com/2012/12/07/stocks-part-xiii-withdraw...