If you want the best big rig sound, you can't beat 5". You can buy a whole 5" turbo back kit from ebay for around $300. With your 12v, as long as you run a tailpipe you shouldnt run into any drone issues. If you happen to think it is too loud you can always throw a muffler on it. 4" will sound good too, but just not as good as 5"

 

I'd reccomend a fuel pressure gauge as well. Makes diagnosing problems much easier.