The 4-headed "shin" on the Tefillin is an unusual variation of the modern Hebrew letter shin, which ordinarily has only 3 heads. There is also a 3 headed shin on tefillin, with the same calligraphy. It appears nowhere else in Hebrew literature, And, the explanation, according to Maimonides, is that is is one of the 31 alleged Halachot that were given orally to Moses at mount Sinai, but not committed to writing (until the Oral Law became a book).
https://www.ou.org/torah/mitzvot/taryag/31_halachos_lmoshe_misinai_according_to_the_rambam/
In fact, apart from this list of 31 halachot, many others are also claimed to have been given to Moses but not mentioned in the Torah. It is interesting - how does one verify philosophically such a claim?
The problem is that there is no written contemporaneous evidence of there being an oral law or these specific halachot being given to Moses. The claim occurs some 1150 years after the event.
This particular claim, of the style of the shin, its calligraphy etc. can be refuted. That is because the Hebrew script at the time of Writing of the Torah was not the same as the one we have today or at the time of the Mishnah. The Hebrew script of the Bible was paleo-Hebrew. And the paleo-hebrew alphabet does not look the same as the modern square letters , which are in fact Aramaic.
The second last letter of this alphabet looks very much like an English "w" and that in fact was a Biblical shin.
Had the halacha of this shin (and indeed tefillin ) been given to Moses at Sinai, it is unthinkable that it would be written in the form of a future vulgate language. It is likely that the scholars who concocted this story of the 31 or so laws were unaware of the history of the Hebrew language.
