ya, you can install a higher resistance rated speaker to an amp, and it will play. Your amp will not push out as many watts as compared to a lesser resistance load, but it will play.
The problem arises when an amp is not stable at a lower resistance load. If you hook up a lower resistance load to an amp that is not stable at that load, it may damage the amp permanently.
8 ohm sub to car amp is okay.
Thanks everyone! I'll just grab a cheap (hopefully not too cheap) 200-300W RMS @ 4 ohm amp to power it then. They've been sitting in my girlfriend's car for like a year now with everything ready to go except for the amp. Maybe now she can get a little bass in her car. Thanks again!