But with less volume. 4 Ohm speakers are used in a car mainly because you get more sound from them than 8 Ohm jobbies due to the 12v feed, this was sidestepped when they invented SMPS but that was the original reason.
^^^^^Considering you can pick up some custom fit Alpine speakers for only £25+ I think you would get a better sound than hacking the car about to fit something designed for another use