The fourth son is Judah, from the root hoda'ah, "thanksgiving" (Genesis 29:35). Leah speaks one of the most remarkable lines in the entire matriarchal record: This time will I give praise before the Lord.
Three sons she had explained in the language of her own pain — my affliction was seen, my voice was heard, my husband will be attached. With Judah she stops speaking about herself. She simply gives thanks.
The Targum Pseudo-Jonathan completes the sentence with prophecy. From this my son kings shall come forth, and from him shall spring David the king, who shall offer praise before the Lord.
Leah has seen David. She has seen the sweet singer of Israel, seven centuries before he is born. She has seen the line of Judean kings who will sit on Jerusalem's throne. She knows that her fourth son is not merely one more tribe. He is the royal tribe, the line from which the messiah will come (Genesis 49:10).
This is why the Jewish people are called Yehudim — descendants of Judah. The whole nation carries the name of a son whose mother received more than her share and decided to respond with hoda'ah. Every Jew who says modeh ani in the morning is, in some sense, completing Leah's sentence.
The Talmud (Berakhot 7b) says Leah was the first person in history to properly thank God. Everyone before her had asked. Everyone before her had bargained. Leah looked at her fourth son and said: enough asking. Enough counting. Now we give thanks. And the tribe born at that moment became the tribe of kings.
The takeaway: gratitude changes the future. Leah stopped keeping score, and her son became the ancestor of every David and every Solomon.