"And you shall bind them as a sign upon your hand": one scroll of four sections (Shema, Vehaya im Shamoa, Kadesh li and Vehaya ki yeviacha). For it would follow since Scripture prescribed both hand tefillin and head tefillin, that just as there are four (separate) head compartments (totafoth) for the head there should be four (separate) compartments for the hand. It is, therefore, written "And you shall bind them as a sign upon your hand" — one scroll of four sections. — But why not say: Just as one scroll for the hand, so, one scroll for the head?
And it would follow, since Scripture prescribes both hand tefillin and head tefillin, that just as there is one scroll for the hand, so there should be one scroll for the head! It is, therefore, written (of the head tefillin) four times "totafoth" (for four separate scrolls).