The workers were poisoned by toxic gases, including ammonia, hydrogen sulfide and carbon monoxide.
Local officials said the accident took place yesterday at Shengda Paper Industrial Company Limited in Zhanggou township.
Qiao Bencai, a temporary employee, was sent down first to the 2.8-meter-deep sink. He passed out while cleaning the waste pulp, while three others who went to save him also died.
Waste pulp and garbage in the sink, which had been neglected for three years, were soaked by rainwater and triggered the suffocating gas emissions, Xinhua news agency reported.
The paper mill managers have been held and are in police custody for further investigation.