U can change out the front speakers as I'm doing also.....but I noticed when I press my bal./fade button I only get balance not fade
But I removed the radio and tested an I do get voltage from the four extra outputs but I have no fade.....please test on your radio please let me know
Just so u know the truck does not come wired for the rear speakers and also at the radio they are missing but not a big deal because if u look at the harness u can cut some of the plastic away and install the wires directly which iam planing to do but I don't have fade control which is weird
But back to your question four speakers is doable but no sub without a amp ....