I posted this in another thread but its good info...
the folks over at r/c heli base have an white paper on the operation of a normal r/c controller... from this paper i found out that nearly all 3-7 channel non computer radios use the same encoder chip... so that means that your 4 channel ground radio can actually transmit 7 channels of data, all you have to do is free the extra input pins on the encoder (they are grounded) and attach your pot and voltage reference and you now have an 7 channel radio!!
here is the link
http://www.rchelibase.com/radio/index.html
btw ive looked in my radios and it seems that to retune one you would only need to replace the crystal, maby a few coils and adjust a few trimmers if i can get my hands on an frequency counter i may try it myself