My first two planes were 3 channel birds: GWS Slow Stick followed by the Great Planes BLT.

Third plane was the 4 channel GWS Estarter and I can't say that I had problems with the transition to 4 channels. On a three channel plane, the rudder is set up on the right stick anyway, so the only thing you need to learn when you fly 4 channels is to ease up a bit on the right stick in a turn to stop the bank and add a bit of rudder to coordinate the turn.

I learned on my own. Lots of time on a simulator and then the Slow Stick on dead calm days.

Good luck!