BSW seems pretty confident that their 4 ohm speaker solution won't kill the OEM amp. How they would warranty that would be another question. Another issue that I see when driving a 4 ohm speaker with an 8 ohm Amp (impeadance mismatch) is that you won't get maximum power delivered to your load (speaker). You'll get a double whammy here. First, your 4 ohm speaker won't be as loud as it should when driven by an 8 ohm Amp because only half of the power actually gets delivered to your speaker. Second, your Amp will heat up more because half of that power that's supposed to be delivered to your 8 ohm speaker (now replaced with a 4 ohm speaker) is now being dissipated as heat in your Amp (thus shortening it's life). To get maximum power delivery, your source and load impedance must match. So, even if you add in a series "load" resistor (trying to match your amp), then half of the power will be consumed by that resistor, which turns it into heat and your speaker would still be only getting half of the power from the amp. Therefore...adding the "load" resistor will only help prolong the Amp life, but will still make the overall experience (sound-wise) pretty bad...depending on what's exactly is in that "load" resistor that may introduce other distortion factors.

To make a long story short, find an 8-ohm speaker