Down pipes and exhaust modification?

[Jules-C]

Pipes 1 and 3 connected on one side of 4 - 2 - 2 and pipes 2 and 4 on the other might give the sound you want but it might make very little difference.

Typically longer header pipes will be better for lower revs and shorter better for higher revs.  This is what some of the exhaust sysyem valves effectively do to improve performance across the rev range.