Dual 2.0" pipes are not the same as 4.0". You can't just add the diameters. a 2.0" pipe has a cross-sectional area of 3.14sq.in. (Pi(r)^2). Dual 2.0 inch pipes have a cross sectional area of 6.28 sq.in., which is the same as a single 2.828" pipe.

2000 Mercury Cougar V6 ATX. 16.0@87MPH, 155.0 FWHP