Mostly about $250 difference.

4PL is a good medium performance radio on FHSS (Frequency hopping Spread Spectrum) and comes out of the box with nice crawler features like dual ESC mixing and 4 wheel steering. It has all the adjustments needed for racing. It's a nice radio for the money and receivers are reasonably priced from $35-50.

The 4PKS and now the 4PKS-R is Futaba's top of the line radio and has all the bells and whistles of the 4PL and much more now that the -R has all the crawler mixing functions. It works on FASST ( Futaba Advanced Spread Spectrum Technology) and is faster than it's lower priced brother. It also has a drop down wheel and can be set up for a lefty. The receivers are around $100-120.