it might be doable with 4" the biggest isue being groundclearence.
IMHO i cant say anything about decibels but i can say that i know of 2 first gen camaros with very very mean largecube bigblocks that are surprisingly quiet.
the first one is the all famous Värlsmästar camaron,yes its a very humble name for what used to be the worlds fastest streetcar.
there might be some pics of the exhaust on this page:
there is also what is known as the nightshift camaro can bee seen and heard in this video its the blown White camaro
[ame="https://www.youtube.com/watch?v=0LHPdTWv_xM"]Stockholm Open 2012 - YouTube[/ame]
both runnning basicly the same kind of exhaust. BIG dual exhaust to above the reaend where it splits into 4pipes feeding 4 round 3" straight thru mufflers(no they are not anywhere near bullets they are apple mufflers with a fullbody size17½x6 1/4"body). surprisingly quiet under low load and still not all that loud under power. hope it helps alitle even if the samples are chevys and might not be best known for there legal racingcareers.