RE: 3ch going to 4ch
Getting a small plane or something bigger depends upon where you fly. A big plane will be more stable, but you should also have a bigger flying area. If you are limited (i.e. only have a small area to fly in), then I think a small plane is easier. If you have a lot of room, then a bigger plane should be easier to fly.
When going from 3 to 4 channel, I think the best way to do it is put the ailerons on the channel where you previously had the rudder (assuming your previous 3 channel plane was throttle, elevator, rudder). Aileron should be the primary means of turnign a 4 channel plane, with rudder just helping to coordinate and smooth out turns. I usually fly with mostly trottle, elevator, and aileron, and my radio mixes a bit of rudder in with the ailerons.
The hobbyking Bixler or Bixler 2 gets good reviews and is pretty cheap.