said by Draiman:
Another option is add a 2x2 or another 2x4 onto the existing 2x4 thus extending it to add more fiberglass insulation. Yet another option is run a ton of empty conduit in the wall before you foam. Personally I'd extended the framing with another 2x4 to give you 8" of space then fiberglass with facing towards the inside and drywall over that. Cheap, simple, and effective.
Strapping a 2x2 over the existing 2x4 is a good idea, but what I'd do is spray 2" of 2lb foam into the wall cavity first. This create a vapor barrier at the exterior wall sheathing - 2" of 2b. IS a real vapor barrier according to all tests done by manufacturers & testing labs. Then install the batt insulation over that. This gives you about R26-28 in a 6" nominal wall and he ability to easily fish wires at a later time.
Do not install a poly vapor barrier over the batt insulation in this case - there should be one and only one vapor barrier in a wall, and that is provided by the 2" of foam sprayed against the exterior sheathing.
You can read more about 'hybrid' insulation like this at www.buildingscience.com