Four more human cases of the West Nile virus have been confirmed, bringing the total to eight in the state this season, health officials said Friday night. Three residents in Middlesex County and one in Hampden County, who were listed as probable cases earlier, were confirmed with the illness and all patients were recovering, officials said. “Confirmation of these four cases is a timely reminder that people need to take the threat of mosquito-borne illness seriously,” said Dr. Al DeMaria, epidemiologist for the state Department of Public Health. The West Nile virus threat level was raised to high in Chicopee, Chelsea, Everett, Lexington, Malden, Medford, Melrose, Revere, Saugus, and Winthrop, the department said Friday.