RE: Converting 3 channel to 4
Flying with ailerons is easier than rudder only in my opinion, you have much more control over the plane. Definitely make sure you have a 4 channel TX and RX before you think about doing anything. I would suggest going with one servo that runs both ailerons, so when looking at the top of the wing from the back of the plane, you turn left, the left one goes up and the right one goes down at the same time and vice versa. Like feihu was saying, if your wing does not have a slight angle or taper to it as it goes towards the tip (dihedral) then it might be a little difficult to put ailerons on it.