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With the asphalt roofing shingles (vapor barrier) on the exterior wall, don't add another vapor barrier (plastic) on the inside, you will have problems.
You could add the foam board inside to the studs but--- it will keep your cavity colder (wetter) because you will stop the heat from the room warming it. I'd rather cut-n-fit the foam on the inside with Roxul in the rest of the cavity, tight to the drywall face to limit convective loops there. Add some foam (fanfold, sill sealer, 1/2" foam board) to the studs for a thermal break from the outside temperatures (wood is R-1.25 per inch) using proper layers to end up with a good fit, no air gaps. http://www.buildingscience.com/documents/information-sheets/installation-of-cavity-insulation/
Limit the air moving from the room to the wall because of pressure differences by air sealing the drywall well. http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/
You lose 100 times more from a small leak than from diffusion. Caulk the sheathing to studs/plates for air sealing once it's exposed. Seal the wiring/plumbing holes in the plates and exterior light boxes. http://www.wag-aic.org/1999/WAG_99_baker.pdf
If in Zone 4, you don't want/need a v.b. at all. http://www.buildingscience.com/documents/information-sheets/info-sheet-310-vapor-control-layer-recommendations/