A Feast for Crows
The status of the fourth Sand Snake, Sarella Sand, is unknown, though according to Prince Doran she is said to be playing some sort of "game" in Oldtown. Sarella is famous for her insatiable academic curiosities, and there are fan speculations that she is actually disguised as a male Citadel acolyte named Alleras ("Sarella" spelt backwards) with the nickname "the Sphinx", who debuts in the prologue of "A Feast for Crows" and helps Samwell Tarly when he arrives at the Citadel. In the ending chapter of "A Feast for Crows", Alleras is tasked by the departing Archmaester Marvyn to take good care of Samwell as the latter starts his maester training.