The 4.1 is louder. The 4 is the trail version with a spark arrestor so they keep the sound a bit lower, I think it is about 96-97db. The quiet core will drop it to about 94-95db. The 4.1 racing pipe is about 99-101db.
FMF Has a quiet core.for the powercore 4.i think there around 14 buck's.i run a Q4 with the quiet core and spark arrester removed.mainly because it run's better.it doesent really get loud till wide open throttle.