The 4PX is compatible with the FASST, as well as the FHSS / S-FHSS / T-FHSS receivers.
I have a 614FF, 614FS, R304SB, and an R203GF bound to mine.
Looks like the GR400F is FASST, and the GR400S is FHSS. If they worked with the 4PK, there should be no problem with the 4PX.