Dan, the fact that you're even *thinking* of running 4 gauge from the alternator makes me nervous (complete overkill). Seems like you're grasping at straws. Though I am glad you're asking here before you put you and your rig at risk
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Wire gauge depends on your peak alternator output. Assuming alternator is OEM and less than 100 amps, less than 5' run to isolator.. run 12 gauge. Solder and shrink both terminals. If we're talking about your V8, that 10 guage will afford you 150 amps peak.
At the very least, fuse the supply into the cab. Add up the potential draw and fuse accordingly. The fuse should be rated for less than the capacity of the wire you run (common sense). Now.. there's one drawback to using a fuse over fusable link. A momentary short will not cause a fusable link to fail.. it will, however, blow a fuse. So to that end, cary spares!
If you have any accessories that are wired outside of the main fuse block, you'll want to fuse those as well. Waytek has some great inline fuse holders that take ATO and maxi fuses (please do not use the 3M self stripping ones!).
The key to automotive wiring is knowing wire capacity and potential areas of fault. The rest is common sense.
Use a gauge -> amp capacity chart to determine *what* to use. The chart should cover run lenths (accounting for voltage drop). Fuse at or less than capacity. Most of these charts are pretty conservative so it's ok to fuse at capacity (i.e. 25 amp fuse for wire rated at 25 amps @ X feet). Any time you raise the temperature of wire, you push it closer to capacity. So there are situations where you need to "over wire". When possible, keep runs short, keep bundles small, and keep wires away from heat.
Fuse prior high fault areas (like the cab.. tons of places for a short). For short, simple runs like battery -> starter, etc, just make sure you *protect* the wire by routing it out of harms way (within reason.. don't make run too long), applying spiral wrap/ split loom to minimize damage from rubbing, fluids, heat, etc, etc.
Feel free to PM me.. I'd be more than happy to supply my cell # so you can call if you get in a pinch.
