The arm tefilla has one compartment and one parchment and the head tefilla has four.
Let's say someone has only one type available. He has two head tefillin or two arm tefillin. In such a situation, may one place the head tefiila on his arm or arm tefillah on his head? Or should he wear only one tefilla - whichever is the appropriate one for that body part?
(Assume that the strapping is done correctly.)
This is discussed in the Talmud (Menachot 34b). Turns out the issue isn't the number of pieces of Klaf at all. Even one piece for the head and 4 pieces for the arm is valid. The issue is the number of compartments.
If he has an unused head Tefillah, he can cover it with another piece of leather to make it effectively one box instead of 4 and then bind it to his arm. If it was used though he can't because that would be reducing it to a lower level holiness. You can't turn a hand Tefillah into a head one without getting a new case that has four compartments and moving over the innards.
