Shulchan Orech 32:44 discusses the hair which is used to bind the parchments in the Tefillin.
Rav Moshe Isserles says: "the custom is to bind hair on the parsha and then bind on that kosher parchment and then bind on that hair. And the custom is to use calf-hair and if one doesn't find calf-hair then one uses cow or ox hair and one washes the hair very well so that it is clean. Some of this hair should be seen outside the Tefillin-boxes."
Rav Aryeh Kaplan in "Tefillin" in the beginning of the chapter "By a Hair" says that there are four hairs which come out of the base of the head-tefillin. [for Rashi tefillin] Does anyone know his source that there should be specifically four?
The commentaries that I saw seem to discuss the length of the hairs and the location of the hairs, but not the number.
I looked at my own Tefillin and there are indeed four hairs. But I looked at other people's and there seem to be differing numbers of hairs.