Would you please explain to me what the significance of the two “shins” on the head tefillin is, and why one has four arms instead of the usual three?
The presence of the shins on the head tefillin has many explanations. I’ll discuss just a few here.
The two shins together spell “sas” meaning joy. We must view the awesome mitzvah of tefillin as a privilege that we are honored and happy to perform. Since the tefillin symbolize Torah and mitzvot, we are reminded in general to learn Torah and do all the commandments with joy. It’s not coincidental that the shins are on the tefillin of the head, the seat of thought. Often, happiness is a result of our state of mind. Re-arranging our thoughts can help us see things in a new light, which may bring us joy. Thus, re-arranging the letters for the Hebrew word for thought - “machshava” -, yields “besimcha” - a state of joy.
Also, “shin” stands for “shefa” meaning bounty. The shins on the tefillin symbolize conduits channeling Divine energy from above to below. In Judaism, this is viewed as an inverted tree where the roots are fixed on high and draw spiritual sustenance down to the physical world of creation. Since man is the pinnacle of physical creation he is like the trunk of the inverted tree, disseminating this influence into the physical world. Therefore the shins are like roots reaching on high, through which Divine energy is brought down and focused in our minds. From there it is channeled to our arm, the instrument of activity. This symbolizes the dissemination of Divine energy into the world through our interaction with the surroundings.
The three and four headed shins are understood as referring to important sevens such as the seven days of the week, the seven-branched menorah, the seven divisions of holy and secular knowledge, the seven patriarchs (Abraham, Isaac, Jacob, Moses, Aaron, David and Joseph) or alternatively, the three patriarchs and four matriarchs.
Furthermore, the head tefillin, referred to as a crown, together with the three and four armed shins having seven points, corresponds to the ten sefirot. These sefirot channel Divine energy from on high and in turn are projected onto our bodies. Thus, the tefillin that we wear are integrated into this pattern of the sefirot in our body. The head tefillin correspond to keter. It rests on our minds, between the right and left hemispheres of the brain, corresponding to chochma and bina, respectively. The knot of the head tefillin is placed at their juncture on the brain stem, corresponding to da’at. The hand tefillin is tied to either the right or left arm, the place of chesed and gevurah. The box of the hand tefillin rests upon the heart, the place of tiferet. The straps at each side reach the thighs, corresponding to netzach and hod. They fence in the groin, infusing the areas of yesod and malchut with positive Divine energy originating from the head tefillin, which is in turn “rooted” by the shins on high.