פירוש על ברכות 14:5

This time I will thank G-d -Because she had Ruach Hakodesh(a holy vision) that Yaakov was going to have twelve tribes(through 12 sons), and he has four wives, so when she had her fourth son, she gave thanks because she got a greater portion than the original calculation she expected(on average each wife had three sons, and she had four).

Until Leah came, etc. As Rashi explains, and similarly as is explained in Bereishit Rabbah at length, see there. Since the verse doesn't give an explanation for her being thankful for the fourth child, the midrash expounds thusly. And it must be that she was not literally prophesying, for we see that Leah is not considered as a prophetess in the first chapter of Mas. Megillah 14a. Rather, without her knowledge, God put words in her mouth to predict the future. This is similar to what Rivka says in Gen. 27:45, "Why should I lose", etc., for she too is not listed as a prophetess. And similarly we can explain the upcoming midrash regarding Reuven.