This is the basic meaning of the verse, but homiletically it was interpreted to indicate that four categories of people maintain the teachings of the Torah.
According to one version of the Midrash, they are the experts in Bible, Mishnah, Halachah and Aggadah (Midrash Talpiot). Another version enumerates the experts in the four methods of understanding the Torah – the plain meaning, the homiletical approach, the philosophical or rationalistic and the mystical.
The tefillin also seem to hint at the various keys to the Torah. The head tefillin have four compartments, whilst the hand tefillin have only one. This suggests that when it comes to using the head to seek understanding, there are various legitimate approaches that one may follow. But in regard to action, there is only one right way, and one cannot say that, for example, eating kosher and not eating kosher, keeping Shabbat or not keeping Shabbat, are equally valid and acceptable.