I have naked Mk3 and Mk4 chassis in the shop currently. The Mk4 is 2" lower at the rear cockpit wall, 2" lower at the curved bar running side to side ahead of the trunk opening and 2 3/4" lower at the quick jack/bumper mounts. Everything forward of the back wall is dimensionaly the same.

Something to keep in mind if your doors have already been fitted is that when you bring the tail down it will open up the gap at the back of the doors rather significantly.

Jeff