Musar do Wyjścia 28:39

The four paragraphs of the Torah which are written in the תפילין correspond to the four letters in the Ineffable Name of G–d, i.e. י-ה-ו-ה, as explained in the Tikkunei Zohar. The "carriers" of G–ds glory derive from these four letters in His name, the emanation being three or four respectively, as we will explain. This is hinted at by the fact that on one side of the תפילין, phylacteries, we have the letter shin with 3 "legs," whereas on the opposite side, the letter shin appears with four "legs." The origin of that practice is the verse in Exodus 28,39: "ושבצת את הכתונת שש, ועשית מצנפת שש." The Talmud Zevachim 19a says that there was a spot on Aaron's forehead where he laid his phylacteries between the headband, מצנפת, and the golden plate, ציץ, (Exodus 28,36). The word שש refers to the two shins we have on the תפילין.