the Four Classic Novels of Chinese literature, namely: A Dream of Red Mansions 紅樓夢HónglóuMèng, Romance of Three Kingdoms 三國演義SānguóYǎnyì, Water Margin 水滸傳ShuǐhǔZhuàn, Journey to the West 西遊記XīyóuJì
the four elements: earth, water, fire, and wind (Buddhism)
the four freedoms: speaking out freely, airing views fully, holding great debates, and writing big-character posters, 大鳴大放dàmíngdàfàng, 大辯論dàbiànlùn, 大字報dàzìbào (PRC)