Charles, can you please post your stereo system configuration? I have the following theory on this whole 4/8 ohm thing.

Assuming that rear door speakers and rear deck speakers are wired in parallel (I haven't checked this, but I assume they are), Volvo uses 8 ohm speakers in both of those locations to create a 4 ohm load for the amp driving them. In case of 6 speaker cars, like mine (no door speakers), the load on the amp driving the rear speakers ends up being 8 ohms, which is a lighter load and provides the amp with some breathing room. So, my theory is that in a 6 speaker setup 4 ohm rear deck speakers will be fine since they will not exceed the 4 ohms that the system was designed for. In 8 speaker cars, however, the load of 4 ohm speakers in the rear deck and 8 ohm speakers in the rear doors will add up to about 2.67 ohms making it tough for the amp to deal with at high volumes. So, if your system is a 6 speaker system you should not worry about the 4 ohm speakers in the rear deck, but if you have door speakers, you need to either unplug them or realize that you are over-taxing your amp by the resultant load. Of course, this is all for the factory Volvo head unit. If you have a premium system with external amp, things may be different.

Just a little rambling from the gEEk.