The first part of this season continues where the second season ended, and the battle for the Mohist school finds its conclusion. In the second part of the season, new alliances are made and new foes arise. (Source: AniDB)

The fourth season takes place after the Mechanical City has collapsed and the Mohists are almost eliminated. The Qin Empire plans for a period of peace and prosperity and the Emperor utilizes a huge amount of labor to build the Great Wall and plans more sophisticated programs. There is apprehension in the city about the function of the giant boat Mirage which appears off the coast. Various rebel factions came together in Shanghai to defeat the Emperor's ambition. The Emperor and his forces, including the Quicksand group, the Yin-Yang School, Net Trap Organization and swordsman Shengqi are all arraigned against the rebel group comprising the Mohists, Confucianists and the Taoists. Tianming's education and training continue as he is faced with more and more challenges as leader of the Mohists. (Source: Qin's Moon Wiki)

In the second season, Emperor Qin sends Quicksand to destroy Machinery City, the home of the Mohist School. At the same time, Duanmu Rong sends the three kids, Tianming, Shao Yu, and Gao Yu, into the forbidden grounds of the Mohist School, a place where nobody could enter without permission. (Source: AniDB)

Nemeses—powerful and mysterious demonic entities that fall from the sky and vaporize anything they touch. The only ones who can combat these creatures are Sorcerers, those who have survived an encounter with a Nemesis but were infected in the process. Seth, a Sorcerer from Pompo Hills, sets out on an adventure to exterminate all these Nemeses. Accompanying him are Doc and Mélie, fellow Sorcerers who share his ideal. Their main objective is to bring about a world where Sorcerers are no longer persecuted for being infected, and to that end, desire to destroy the source of the Nemeses themselves: the mythical Radiant.