Originally Posted by lgbr
A two channel amp and a four channel amp rated at the same wattage will power four speakers just the same. However, a two channel amp powering four speakers would be running at 2 ohms, so make sure your amplifier is stable at 2 ohms (that means not sony). I'd recommend you stick to a four channel amplifier, it's just a lot cleaner to have one speaker/one channel.
The resistance the speakers pose on the amp is dependent on the way the speakers are wired and what resistance the speakers are themselves. It is not written in stone that 4 speakers to a 2 channel amp will have a resistance of 2 ohms.
In car audio, really the only difference between powering 4 speakers with a 2 channel vs. 4 channel amp is that there is no independent adjustment of the volume between front and rear speakers. That is it, nothing else. There is no problem in running 4 speakers off a 2 channel amp, other than not being able to fade the music to the front and rear. Remember, music is recorded in stereo, not surround.