The Four Symbols refers to a group of five guardian spirits. They consist of Meng Zhang, Ling Guang, Jian Bing and Zhi Ming. Each of them represent one of the four cardinal directions and the seasons. As such, they govern and maintain Aileron's state of nature as the seasons change. It is said that their sacred ground is the small island known as Horai Island.
Also known as Qinglong and Seiryuu, Meng Zhang is the Azure Dragon of the East and a wood spirit. He maintains power and control over the season of spring.
Also known as Zhuque and Suzaku, Ling Guang is the Vermilion Bird of the South and a fire spirit. He maintains power and control over the season of summer.
Also known as Xuanwu and Genbu, Zhi Ming is the Black Turtle of the North and a water spirit. He maintains power and control over the season of winter.
Huang Long is the secret member of the Four Symbols as well as their leader, the Yellow Dragon of the Center and an earth spirit.
Background
The Four Symbols control and protect one of the four regions of Aileron: the northern, western, eastern and southern regions. As they also represent a season, they gain more power during that specific season. The secret fifth member of the group is the Yellow Dragon, Huang Long, who leads them. It is believed that Horai Island is their sacred ground.
Trivia
The Four Symbols are based on the Chinese constellations of the same name.