Typically 4 channel transmitters don't have multi channel mixing, therefore not being able to perform what you're trying to do. You will need to buy an Elevon mixer control unit. Usually runs around $15USD. You are to plug both servos into this gizmo and it will alternately operate both servos using only one control. And yes you are required to have a six channel receiver. Plug both servos into the mixer. Then plug one of the connectors from the mixer into channel 6 of the receiver, and the other one into whichever channel you're using for your ailerons. (Usually channel 2 or 1)

Here is a picture of what I am talking about. Check out the Great Planes Elevon Mixer. This should work well with your setup.

http://www2.towerhobbies.com/cgi-bin...&I=LXELT4&P=ML