Why do you need a Hi Power Alternator because you're putting in a 4000 watt system?
Put in a Optima Yellowtop. If you're really worried about it add an Optima Bluetop in reserve.
Your stock Alt. will do just fine because you will not be using all 4000 watts all at once all the time. Also get a Capacitor.
If you really want one try google.
www.alternatorparts.com