As far as what i have read, the 2" shackle is about as far as you can go in the rear without dearching the springs, removing a spring or two, or do and axle flip.

If you did an axle flip and a shackle flip at the same time and forgo the lowering shackles (which in this particular case would now raise the rear), i thinck then you get around that 4" mark.
Thats how Scratchinfortraction did his.