They said a 38-year-old man who had admitted himself to the hospital in Sapoca, northeast of the capital Bucharest, entered a treatment room and attacked other patients with a transfusion stand.
Three of the patients suffered head injuries and died at the scene while a fourth died later in hospital, the reports said.
Among the nine injured in the attack, two were in a coma, they added.
“Everything happened in less than a minute,” hospital director Viorica Mihalascu was quoted as saying.
“This patient was admitted on a normal surveillance level. His symptoms did not give an indication that a tragedy was imminent,” she told Antena 3 television station.