Leaving Space Between the 3rd and 4th Tefillin "Wrap"

Question:

Why is it Chabad custom to leave space between the third and fourth tefillin "wrap" around the arm?

Answer:

The first three coils correspond to the three letters of the divine name ש-ד-י (Shaddai). The latter four coils correspond to the four letters of the divine name י-ה-ו-ה (the Tetragrammaton). Because these two groups of coils represent two different names, it is appropriate to leave a little space between them. The amount of space you should leave is not specified anywhere—just leave enough space so that the break will be noticeable.

Rabbi Eliezer Posner