The 4PK radio can use the R603 receivers as well which are intended for the 3PK/3PKS, but the radio have to be switch to C1 Mode, instead of the R604 C2 Mode, the mode and resolution of the two radios were different.

The last time when I have my 3PKS radio, it only have the C1 Mode, so the R604 receiver cannot be use for the 3PK/3PKS radio.