My factory speakers are 8 ohm. Can I replace them with 4 ohm without any problems? Seems like all I can find are 4 ohm speakers. My radio is a 2006 delco factory am-fm-cd player.
You'll need to look up the specs on your head unit. It's probably only stable down to 8 ohms, in which case a 4 ohm speaker will damage it.
To replace one 8 ohm speaker, you could wire two 4 ohm speakers in series to raise the impedance back to a safe 8 ohm level, but then you are cutting the power (which is only 5-15 watts RMS) to each speaker in half.
are you only looking to replace your factory sound, or are you up to upgrading? for a couple hundred bucks you can get sound that's 1000x better than the $50 you'll pay for replacement speakers.
check parts-express.com many home component speakers designed for building your own home speakers will be 8 ohm drivers. otherwise you need GM OE factory replacements, or you need to replace the stereo.
If the amp in the ehad unit is only 8 ohm stable, going to a 4 ohm speaker will damage the internal amplifier circuit, more than likely.