The Talmud does not consider Jacob's "other" two wives, Zilpah and Bilhah, to be "matriarchs":
תָּנוּ רַבָּנַן: אֵין קוֹרִין ״אָבוֹת״ אֶלָּא לִשְׁלֹשָׁה, וְאֵין קוֹרִין ״אִמָּהוֹת״ אֶלָּא לְאַרְבַּע. -- The Sages taught in a baraita: One may only call three people patriarchs, [Abraham, Isaac, and Jacob, but not Jacob’s children.] And one may only call four people matriarchs, [Sarah, Rebecca, Rachel, and Leah.] [Berakhot 16b]
Why not? They had the status of wives and were mothers of some of the tribes. I found only one source that refers to them as "matriarchs", a popular 17th-century Musar work:
Thus I have seen it written in the name of one great scholar that Dovid Hamelech was asking the Holy One Blessed is He to break and destroy Israel’s oppressors in the merit of the twelve tribes [shevatim] who were born to the four Matriarchs. For the acronym of the names of the Matriarchs is the word barzel — “iron”: Bilhah, Rochel, Zilpah and Leah. The merit of the Matriarchs stands by us to deliver us from the harsh judgments alluded to by the word “blood” because these derive from the female aspect of the Sitrah Acharah.. [Kav HaYashar 47:2]
Are there others? Is there a dispute? What are we discussing here? Why does it matter whether a progenitress of Israel is called a "matriarch" or not?
Note: This archived answer only confirms what I found: Only the mystics consider Bilhah and Zilpah as "matriarchs". My questions above are not answered.