The same passages from Torah are written inside the hand and head Tefillin. Each passage mentions the commandment to place Tefillin on the hand and between the eyes.

In the head Tefillin, each parsha [passage] is written on a separate klaf [parchment], while in the hand Tefillin, all four scabs are placed on one klaf. (Shulchan Aruch, 32: 2).

Mishna Brurah explains: from what is written “And it will be a sign [to you] on the hand” (Shemot 13: 9), one sign is heard, and since this is one sign outside, so there is only one sign inside, so you should write hand tefillin on one klaf. But the head tefillin, since there are four compartments for the klaf [in the bait], it is necessary that each parsha be written on a separate klaf. Since about the head tefillin it is written: “And they will be letotofot [pl. number] between your eyes.”

Rashi quotes the Talmud (Sanhedrin 4b), the word totofot is a combination of foreign words that mean “four”, which speaks of four divisions in the head tefillin.