The "other two" mothers (Bilhah and Zilpah) were the servants of Rachel
and Leah. They married Yaakov Avinu only in order to have their children
"credited" to their mistresses. This is similar to Sarah arranging for
Hagar to marry Avraham Avinu. However, in this case it worked out and all
the children merited to be Bnai Yisrael.
As a result only Rachel and Leah were considered "full" matriarchs.