Michael,
I believe 4 guage wire is way too bulky to run under the dash and unnecessary. Most car manufacturers use 10 gauge, but I like 8 gauge because of my big lights, two heaters and all the computer gear. It is important to note that the only circuit that bypasses the ammeter is the starter. It pulls over 100 amps and will fry an ammeter. I use 0 gauge welding cable for that connection, directly from the battery to the starter solenoid, then another 0 gauge from the other solenoid connector to the starter. At the standard Ford starter solenoid battery terminal (at the same connector the battery heavy wire is attached to), I run an 8 gauge wire to the ammeter negative terminal. The plus ammeter terminal gets another 8 gauge wire directly to the alternator. I run another 8 gauge wire from the ammeter plus terminal to the fuse block to run everything else. A diagram would help but I don't have one. This simple stuff is all in my head
I hope this helps,
Paul