1. use a ground wire that is the same size as your power wire, so in your case go with 4 guage.

2. you can bolt/screw the ground wire to almost anything that has a clean metal surface. However it is best if you can bolt it directly to the chasis. My grounds are connected to bolts behind my rear seats, and they've been fine there for about 2 years now. Just make sure that there isn't any paint, rust, or anything else exposed on the area you will be connecting the bolt to (you can just sand off any paint that may be there).

3. The speaker wires go directly from your amp to each of your speakers.

4. As for putting your amp under your seat, I would only do this as a last resort, mainly because of theft issues. If someone ever broke into your car, the glove box and underneath the seats are gonna be where they look first. I'd say stick with putting the amp on the backside of your back seat, you won't lose any trunk space that way.