You can use the 4gauge wire already going to your trunk (assuming you don't have the biggest amp I've ever seen. Run your 4 gauge wire into a "distribution block". From there, reconnect your amp. Get yourself an inline fuse holder and hook that up to the distribution block and run it to your inverter.
In my setup this would work. You should calculate the amperage that your amp is drawing and the amperage that your inverter might draw too. I run a 60amp fuse for my 4 gauge wire to the trunk. my amp only pulls about 30 - 40 amps. A 200 watt power supply would pull about 17 amps. from the battery at peak power.
If you post some information about your amp, inverter and PC power supply types and wattage I could give better estimates for you.