I would assume that the factory headunit would be 4ohms all around. I know higher end models can toggle between 4 or 2 ohms; but not mix the two, unless the rear channel is used specifically as a subwoofer channel.
I was floored that this headunit doesn't even have a mid-range EQ setting! That would help alleviate the muddiness from the front door woofers.
Crutchfield doesn't seem to know there are rear speakers at all for this car!