Hi Malcolm,
You can ,in your copy, redefine the wing type to 4 Ail.
That will then give you Ail,Ail2, Ail3 and Ail4 to assign.
Or
Assign 2 channels to Ail,, and 2 channels to Ail2. If you do this you must make sure that the two on Ail, are on the same side of the model for the diff,, to work.
Mixers are a bad way to set up 4 ailerons.
Brian