I have the 190CXPRO3 and love it. I have the 804RC2 head, which I like except with the 70-200 2.8 IS. I had difficulty getting the head to 'set' where I wanted it. It wasn't a matter of slippage once it was set, but maintaining the position I wanted in the tightening process. I added the 488RC2 and that fixed the problem.

Regarding the RC2 and the RC4 - I think it is just a different in the switch plate, but someone else may have a better answer.