Sorry for the noob question, but my 4channel amp (Exile S400.4) has a switch labelled "2chan/4chan mode" which isnt explained in the manual.
What is it used for and what does it do? My first guess would be to take the front L/R input signal and send it to both front and rear outputs, is that correct?

 

That is correct. Set it to 4 channel if you want to use the front and rear preouts from the head unit, that will allow the fader to work.