I'm sure the fifth servo would be aileron - i.e. one in each wing. Simplest setup is just use a y-lead out of the one aileron channel in the receiver, OR alot of modern computer radios alow you to use 2 different outputs in the receiver.

Using 2 outputs can allow more ease of individual trimming of the ailerons, plus get differential (mote up than down aileron movement) more easily via a transmitter function.

If you set it all up ok, a Y-lead is still fine though, and simple.

Cheers,

Cam