The 4th is not a member of the three legendary sannins. The blonde kid in the intro and in the flash backs is a GIRL (wasn't it pretty obvious?). She is the 3rd legendary sannin of leaf village. If u remember the episode about lee when Gai asked them about their dreams, Tenten said that she wants to be a powerful female ninja like the famous Tsunade-sama -the 3rd legendary sannin.

The 3rd students are oro, Jiraiya, and Tsunade. The 4th is Jiraiya's student.