Aye, very certain that the rears (and the door speakers, for that matter) are 4" units, as I replaced mine a few months back. The rear deck cutouts are large enough to accomodate up to 5.25" drivers easily and maybe 6.25" units - depends on the frame of the 6" and you may have to drill some new screw holes.
It'll probably sound like the very best boom box you've ever heard. Good high end and midrange, with as much bass as it's possible to get from a 4" cone.
The subwoofer already has it's own amp - it doesn't use the amp module that's under the CD changer. The sub amp is mounted on the rear deck near the sub. If your head unit has a seperate sub out or a full-range line out, you would connect that to the sub amp's input. If it only has powered outputs, take it to an installer shop and they can hook you up.
sluggo