The first is going back to the origins of the 4% number in the first place, the trinity study. The parameters for that were a 30 year retirement period, and success was "not completely run out of money after 30 years, 95% of the time". If you extrapolate from that original study, if you retire for more than 30 years (FIRE includes retiring early), success drops from 95%.
You can use a variety of online calculators to back test a 4% withdrawal rate - maybe 80% safe, but 20% of the time you'll go broke before dying.