The 4GRS is internal the same as the 4PL. The 4PM is the follow up of the 4PM supporting the faster T-FHSS-SR mode and is now followed up by the 6PM which also had the new F-4G transmission mode. If you look around you probably can find a new or hardly used 7PX(R) for a midrange price which is the bigger brother of the 4PM.

Popular is the Sanwa MT44 which is now I believe followed up by the MT5.

And yes, getting more popular is the NB4 although I have seen some racers changing again after a year.