I am having a hard time reconciling 4:22 with any NORMAL sector times for 146/RJ's.
And yes, I know they are not identical.
My recollection is that 146's operating BME - PER (about 2:40) needed to run reduced baggage and make the occasional re-fuelling stop at Port Hedland, depending on seasonal winds / weather.
How that translates or progresses to an acceptable FP of 4:22 seems to be a giant leap of faith, even for the later aircraft.