Four Seas refer to the territory of China or the entire world. The ancient Chinese believed that China was a land surrounded by Four Seas – the East, West, North, and South seas. The term suggests what the ancient Chinese conceived to be the map of China and the world: Nine zhou (regions) were located at the center of tianxia (all under heaven). Tianxia consisted of nine zhou and its surrounding Four Seas. China was within the Four Seas, while foreign lands were outside the Four Seas. In ancient China, Four Seas referred to all under heaven in most cases, and did not denote a specific body of water. Therefore, the term was used sometimes to mean the seas surrounding the land, and sometimes to specify the land surrounded by the Four Seas.
A man of virtue always does things conscientiously without making any mistakes and treats people respectfully and appropriately. Then all within the Four Seas will be his brothers. (The Analects)