I have the 4PKS and love it. I gave my son my 3PKS so I needed to think receiver compatibily for 12 cars..... That and the fact that Futaba has always made great radios over the years made me stick with Futaba. I still use a 3PB PCM radio when we bring a friend and it still works perfectly despite being 22 years old ! If you buy a 4PK-R you won't be sorry it's a great radio.
You won't regret the M12 either, I had a M8 and it was the best radio out at the time in my opinion. My first radio was a XL2P, which also still worked fine after all those years until it was stolen. Probably still works, Sanwa also makes a great product.
I would only spend my hard earned money on a Futaba or Airtronics system so for me it would be a tough choice if I had no compatible RX. Switching brands with a number of RX to replace is an expensive proposition.