I have 3.5" pipes with a pair of Patriot augers (
http://www.summitracing.com/int/part...3843/overview/) in each pipe behind my 460. Noticeable tone at idle, a bit of a bark when hard on it, but mellow out nicely at steady speed.
BTW, Patriot also has 20" & 26" side pipe turnouts in unfinished, black paint, chrome or ceramic coated, with or without louvered inserts.
Patriot inserts are available in 3" & 3.5", if you're looking for 4" (at a much higher price), check out
http://www.spiralturbobaffles.com/