Changing a 2ch radio to use for 1/2 A plane
You probably won't be able to connect a 4 cell pack to the usual battery connection point that's meant for 6 or 7 cells. The battery elimiator circuit, which is just a regulator, will drop some voltage and that could be a problem.
You may have to "inject" the 4 cell power into the servo connections. normal battery harness plugged into one of the unused servo outputs would take care of that.
But first you'd better check to see if any of those outputs works off the "throttle" stick. If it's hard wired to an internal ESC only then this is all a non starter. Or does the ESC plug into one of those outputs? If so then you're all set.
Does the throttle stick on the transmitter allow for centering springs to be added? It's going to be hard to fly if there's no center return.