FatFIRE is for people who want to live large off their savings (usually understood as >$100k/yr passive income, which generally requires $2.5M+ in invested assets). That doesn't seem to describe OP's position. If FIRE is an interest, then https://www.reddit.com/r/financialindependence/ is a better bet.
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%.
