Bilhah is called Rochel’s shifchah in Bereishis 29:29, and Yaakov’s Pilegesh in Bereishis 35:22.
Mesechtes Brochos 16b: “Only three people are referred to as Patriarchs; and only four people are referred to as Matriarchs.”
In Kabbalah it says that the Imahos were from the Malchus of Atzilus, and Bilhah and Zilpah were from Malchus of BiY”A.