4% has something like a 95%+ success rate over any 40 year period in history.
Many point out that this only works for a single anomaly when you look at stock markets worldwide. Sure 4% has worked for a US investor but IIRC the more successful world wide SWR is 2%. Its a big question as to whether that anomaly will continue another 30+ years. Still its a decent initial goal that can be refined as you approach your potential retirement age