Can you hook 4 speakers up to a amp,and then leave the 2 other...

I wouldn’t run any factory speakers on an aftermarket amp due to poor quality of the speakers from the factory plus they are almost always lower impedance than aftermarket speakers and unless your amp can go down to that impedance it will just go into protect. But say you put 2 aftermarket speakers in the rear and hooked them up to an amplifier if the audio source is still the stock radio you will need to use a line output converter or make sure the amp you use has a high level input option and take the signal going to the stock speakers you’re replacing (most people find the rear speakers are easiest to get the signal from but depending on your car it could have a stock amplifier and alot of car manufactures have a cross over also built in and if this is the case then you’ll need to also get a DSP or bass restoration device because any low frequencies are rolled off and your speakers will sound clunky due to impedance difference from the factory speakers it’s built for and a subwoofer will just be a big speaker and have no bump) and run that into the amplifier then hook new speakers up to the output of the amplifier and make sure the amp has a high pass filter and it’s turned on then with all the power cables ran you’re good to go.