It can be done. I haven't done it yet, but I've seen how 2 fit and know how to add 2 more. The only thing that MIGHT get in the way is the spring for the trunk lid... Personally, I don't think it's worth the effort. You could go with a 10" shallow mount sub and achieve better results and it won't add too much weight or consume much space. I've had such a sub on a motorcycle and it was being powered by a very small (can fit in your pocket) Rockford 500w amp if I remember correctly. That's assuming you're concerned with weight and/space. I have installed 4 6x9's in a GN before though, just not all in the rear deck. I put the other 2 between the rear window and the quarter glass. Weight wasn't a concern with that install, so I put 2 12's in it too and it sounds great.
