the Four Classic Novels of Chinese literature, namely: A Dream of Red Mansions 红楼梦HónglóuMèng, Romance of Three Kingdoms 三国演义SānguóYǎnyì, Water Margin 水浒传ShuǐhǔZhuàn, Journey to the West 西游记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, 大鸣大放dàmíngdàfàng, 大辩论dàbiànlùn, 大字报dàzìbào (PRC)