I used the passenger side hole too, and it worked well. I placed an 8 gauge cable and it fitted well. I suppose a gauge 4 will fit too, you'll have to try. The other thing you need to know for adding an amp, is do you have the infinity sound system or not. In my 2009, I have the infinity system, so if I tap the sound output of my HU, the signal is of the same amplitude regardless of the volume button, the Infinity Amp is connected to the CANBUS (vehicule information bus) and gets the selected volume from there and then decides the amplification needed. So, to connect to it to get RCA (or line-level signal) you cannot do that, you need speaker-level signal and then go from there. This is the route I chose.
Check your vehicle specification carefully. If you have the infinity sound system, there will be an amp by the driver's kick panel. Look there.