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)