Sir, what exactly are you trying to eliminate with a 4" exhaust pipe, versus the stock system? Once you run engine exhaust through a turbo charger, and a DPF, as well as a SCR NOX filter, there is nothing much to be gained past those pollution control systems, except noise. Zero, nada, nothing. Noise is something children enjoy making, the older and wiser crowd enjoy silence as well as peace and quiet.
Unless you consider increased exhaust noise "performance" you are fooling yourself and everyone else. Then this "for off road use only" post. Another WTF moment... your veracity and intent is lost... FAIL.
You will never reduce exhaust pressure on a turbo charged engine. Why? Because the exhaust turbine side runs at 3 to 4x the pressure as the compressor/boost side. That is back pressure against the pistons, rods, crank, the whole assembly, on the exhaust stroke. So if you are running full boost, 22 psi on the intake side, the pressure pre turbo on the exhaust side will be 66 to 88 psi running against the motor and drive train. There are no free lunches, if you gain boost on one side, you lose it in compression of the other side of the turbo, the exhaust side. Basic law of physics, nothing is free. So you can't make the motor breathe better, pre turbo, and that's the crux of the matter. What happens post turbo, on a VNT turbo design, is moot, in terms of increasing performance. Nill, Nada, Nothing.
Exhaust flow and pressures used to matter on normally aspirated engines, it matters not one bit on a turbocharged engine with a VNT Turbo design.
I would sooner spend my money on a penis enlargement surgery than exhaust modifications if I wanted a chick or a babe magnet or some such sort of compensator with the chick, babes, or hook up monsters, I don't know what you kids call them these days.