I have a 4pk and have no issues with steering speed using any futaba reciever (603FF, FS and 614 etc). A mate of mine has a 4PKS and when he turns down the steering speed using any 603 reciever the steering speed drops to extremely slow with a very slight adjustment. It works fine with a 614 reciever.
Is this normal as I want to upgrade to a 4PKS and pass on my 4PK to my son to use, I just dont want to have to buy more recievers.

Any advice on this issue would be greatly appreciated

Are you using digital servos?

Yes we are both running futaba digital servos in our electric cars and savox digital in our gas cars.
Its wierd that when you turn down the steering speed on the 4PKS with any receiver other than a 614 the speed drops to very very slow straight away.
I hope this is just an set up issue on the transmitter as I really want to buy a 4PKS but dont want to buy heaps of recievers as i already have lots of 603's,
All recievers work fine with my old 4PK.

If it has frame rate settings I would try lowering them when using the older receivers.

I think he needs to change the servo setting mode and rx settings .check manual .i think p 46.

Cheers