(Exodus 13:9) "And it (the account of the exodus from Egypt) shall be to you as a sign, etc." — one parchment containing four sections (of the Torah). It would follow (otherwise),viz.: Since the Torah prescribes tefillin for both the hand and for the head, then just as there are four (distinct) sections in the headpiece, so, there should be four (distinct) sections in the armpiece. It is, therefore, written "and it shall be to you as a sign" — one parchment containing four sections.