Julian Freedman of Institute of Management Accountants wrote:
The head Tefillin has the Hebrew letter 'shin' embossed twice on it. One of them is written normally with three branches, and the other one has four branches. Why?
Tefillin embody the entire Torah. According to the verse, Tefillin enable "the Torah of Hashem to be in your mouth." The two different shins represent the two ways the Torah was written: In stone and upon parchment.
Hashem told Moses to write a Torah scroll using ink and parchment. This Torah was written as we write it today --- i.e., with the 'normal' three-pronged shin. The letters engraved on the tablets, on the other hand, were formed by empty space --- this empty space is legible due to the outline formed by the remaining stone. The four-pronged shin represents the outline of the shin engraved on the tablets. By the way, this is the only instance of a four-pronged shin.
This teaches us that if we learn Torah (the three-pronged
shin of the scroll written by Moses), Hashem will give
us the gift of understanding (the unique four-pronged shin
of the Tablets, which were given as a gift to the Jewish people).
- Hagahot Semag, cited by Beit Yosef, Orach Chaim 32