It is Fasst if you use a Fasst receiver with it. The one it comes with is the R304SB which is a type T-FHHS telemetry receiver but this radio can be set model by model to modulate in all types of Futaba's technologies including Fasst, S-FHHS and T-FHHS.

That means your collection of Fasst receivers will still work with the 4PX.