Rabbi Joseph B. Soloveitchik points out that the names of Leah’s first three sons reflect her attempts to get Yaakov to love her as much as he did Rachel. After three sons, Leah finally got the message that she would never supplant Rachel as Yaakov’s most beloved. Therefore, with the birth of Judah, her names cease to reflect such attempts and are indicative solely of her gratitude to G-d. (From Echoes, cited in the Mesoras HaRav Chumash.)