in my infinite wisdom when purchasing an amp kit for my sub amp i did not consider the fact i was going to put in a 4 chan amp for cabin and bought a 4 gauge wire since thats all the sub amp requires, so my question is will an 8 gauge wire fit under the door sill cover with a 4 gauge or should i just get a 0/2 and run a length of it down the door sill?

 

A single run of 4g from the battery is likely large enough for both amps. Get a distribution block and call it a day.