Hi Metalhead,
Can I ask a few questions?
What are you planning to spray? Paint, lacquer, automotive 2k, water based?
Is that a gas powered compressor? 4HP on 15gallons seems like a big motor unless you're listing the "peak HP". My 60 gallon compressor is 3 or 3.2HP running.
Either way, for guitars, I think you'll probably want a mini-gun, and most of those use under 6cfm. You'll also want a moisture filter, but those are easy to get at a great price off E-bay. Don't buy one of the "buy it now" filters if you do this, hold out for a Norgren or such. I've got $300 in filters now, but paid $35-40 total with shipping
Last thought, there are a few people here who may chime in, but you may want to search over at kustomkulturelounge.com too. But be forewarned, half of them will tell you to buy a Sata or an Iwata for a gazillion dollars, because they make their livings with their spray guns.
Regards,
Todd