I think it has to do with how it’s dispensed. If the person before you bought regular fuel, the hose still might have residual fuel from the other kind, meaning you could end up with a different ethanol level than expected. That’s a fire hazard. 4 gallons is probably overkill but better safe than sorry.
Edit: basically, if you accidentally get half a gallon of 10% ethanol fuel and half a gallon of 15% ethanol fuel, you actually have 12.5%.
