You could fly a long time on 4 channels but sometimes it is nice to have more capabilities. Many 6 channel radios offer function "mixing" that allows for flaperons, elevons and coupling one control to another to make automatic compensations. You won't need this in a trainer but one thing to look ahead to is using the same transmitter with more than one plane. This requires what is called "model memories" so that you can store plane set=ups and trim settings for each plane and select which plane you are going to fly from an on-board screen. You would only need to buy the airborne portion of the radios. The Futaba 6EAXS is an example of this and I BELIEVE the Focus optic 6 has these features but please verify this. All major brands of radios have radios with many features available. Your LHS will be happy to show you some of these. Just remember - they are all "used car salesmen" in disguise. Gosh I hope I'm wrong about that, I guess I'm just saying keep an open mind.

Extra channels for retracts, flaps, bomb drops, lights, smoke, brakes(big planes) or just about anything you can wish really.