Here is a copy of the install spec with our kit. When we bought it is was listed as a series 6 kit.
It shows 5 inches on the drawing, not 4 inches as you are indicating.
We actually made a template out of aluminum to install them.
The dimension is not measured along the surface of the wing but it is the horizontal distance measured from the tangent at the leading edge. We actually have them set at just short of 6" back. This is the leading edge of the VG's not the centerline as listed on the drawing.