4 ohms of courseyou need less power for better performance8 ohms for home use only
The impedance of your speaker has to be matched to the output impedance of the amplifier.If the speakers impedance is to low it can be damaged over time or be blown completely.If it's to high you risk damaging the output transistors on your amplifier.Matched impedance - best performance.
manofmud has it right. check the documentation for your system and use what it suggests. Soko suggest 8 ohms is for home use only, but I've found the 4 ohm is more common on home stereos. If you hook up 4 ohm speakers to a high watt stereo or amp, you will end up getting a lot of distrortion at higher volume.