The Aztec fourth sun, or era, was Nahui Atl, meaning “Four-Water.”
Nahui Atl was the fourth age in the Nahua creation tradition known as the Five Suns. Each era was identified by the number four, or nāhui, followed by a day-sign from the 260-day ritual calendar. In this sequence, Nahui Atl followed Nahui Quiyahuitl, the Four-Rain era, and preceded Nahui Ollin, the current Fifth Sun or Four-Movement era.
The fourth age ended in a catastrophic flood. According to the best-known version, the inhabitants were transformed into fish, while one couple survived by hiding in a hollowed cypress or tree. This watery destruction explains the name Nahui Atl: atl is the Nahuatl word for “water.”
Readers may encounter a related answer, Chalchiuhtlicue, in summaries of the myth. Chalchiuhtlicue is the water goddess who serves as the sun or presiding deity of the fourth age in one common version, while Nahui Atl is the era’s calendrical name. Therefore, for a question asking for the fourth sun or era, Nahui Atl is the precise answer.