The rear speakers are pretty useless but still vital to sound stage fill. Your main definition and clarity will come from the door and dash speakers and the sub (you should have one if you like audio). The rear speakers just need to be good enough to fill the stage and keep the bass from feeling like it's all coming from the back. Decent 4" speakers can do this. You'll want better if you carry passengers in the back (like if you're a tiny Asian and so are your passengers
), but otherwise, just go with good type-s 4" or the Infinity or Polk equivalents. No need to spend big on custom enclosures and such for those speakers.