All I can tell you is to forget it and buy one of the shelf. Transmiters must be very reliable since from them depends the safety of your airplane and everything near it :shock: . A 4 channel transmiter/receiver is not as easy to build so chances are that a schematic for all that work will not be freely available :cry:
If you are trying a "small" electric airplane you might be able to go by using a old R/C car and its transmitter and reciever. One uses to control the motors and the other one to control the turning. No elevator, nor rudder :roll: 2 motors should achieve control by making one go faster than the other one to turn.
So you are better off with one transmiter used for that exactly R/C airplanes....