Well i can say for definate that you can cange the core of the mk5 rear boot lock into the mk4 lock. See pics below how i did it

The core removed from the old lock by removing two c clips from the rear


The new core beside the old lock. Had to pry the front cover off the mk5 lock to get at the core


Putting the first c clip back into place


Spring, lever and last c clip laid out ready to put back together


All done