The only place where a BOV is usefull is on the drag strip, no horsepower gain whatsoever, just takes out a little turbo lag and betters your time a little. Little Impreza boy's don't know that, they just put it in because it sounds like that idiotic film called 'Fast and the furious'.
Going for 4" exhaust is cool but you won't find any noticable difference from a 3", the 4D56 just doesn't breathe that heavy. 3.5" downpipe to a 3" exhaust with a good resonator would be a sweet and practical setup. The big problem with wide exhaust in the Pajero is making the bend over the rear axle, anything above 2.5" kinks unless you have a really skilled exhaust guy with super equipment.