4ch amp got 4channels, front left and right, rear left and right. if running front speakers and one sub, they will combine the rear 2 channels into one (called bridge mono) to run the sub, and the front ones run stereo as per normal.
if u have 2ch amp only, its advised to let it run just the sub. the front speakers u can drive from your HU internal power.
if u have 2ch amp only, its advised to let it run just the sub. the front speakers u can drive from your HU internal power.