1. What 4 channel amps can I use with the factory radio?
anybrand, any model

2. How would I hook it up properly?
your stock headunit doesn't have RCA outs, so you'll need to get an "inline" RCA converter. you attach this to your wire harness and it'll give you RCA's.

3. Where can the 4 channel amp be located in the vehicle?
I suggest under the passenger seat. That's a chair that's never removed and should have enough room to still be completely functional.

3. How would the subwoofer amp receive the audio signal?
The amp's remote in will be fulfilled by the "power antenna" wire from the headunit.

4. Would there be any risks to using 2 Amps?
Your battery and alternator weren't designed to distribute the needed power. You'll have to upgrade before going over 500W total.

I honestly think you should just upgrade the HU. It makes a 1000% difference when reaching for quality. Also, you won't have to hack and guess about what wires to tap and use when connecting the amp. The stock headunit harness is completely grey, it's difficult to tell any wires from another.