ORIGINAL: Longhair
The one thing I don't like about the 4PK over the 3PKS is there is no FASST - just built on 2.4GHz.
The 3PKS is backwards compatible with AM & FM as long as you get the right module for it.
Just to get things straight here. The Futaba 4PK uses the same 2.4GHz FAAST system as the 3PK. You can use the Rx for the 3PK with the 4PK.