Doctors discovered on Tuesday that three men at the nursing home in the northeastern province of Heilongjiang had had either one or both of their testicles removed with a blunt razor, the official Xinhua news agency reported.
The fourth victim was not found until later after police carried out a full inspection of the facility, the news agency said.
The victims are in hospital, though their injuries were not life threatening, Xinhua added.
"The victims identified the perpetrator as a worker at the nursing home, while a staff member at the home said the man, in his 30s, was also housed there for mental illness," it said.
Police have not identified the suspect or said why he carried out the attacks, Xinhua added.
China's vast and underfunded healthcare system has long be plagued by problems including corruption, poor training and attacks on staff by patients unhappy with poor treatment.
Providing affordable, accessible healthcare is one of the key platforms of President Xi Jinping's new government, but the healthcare bill is set to hit $1 trillion by 2020, according to a report by consultants McKinsey & Co. REUTERS