Adding a 4 channel amp is a great idea. Very nice! It means 2 channels are sharing the workload of one speaker between them and therefore and drive it with more power. A 4 channel car amp is basically an expanded version of a 2-channel amp. This saves installation space and makes it easier too. Whether you have a factory stereo or a great aftermarket non-factory one, adding an amplifier is one of the best decisions you can make.