Hamilton General Hospital declared the outbreak today in its surgical trauma unit.
Three people on the unit and one patient elsewhere in the hospital have been infected with the intestinal bug.
A patient infected with C. difficile died Monday at a different Hamilton hospital, but officials say the bacteria may not have directly caused the death.
St. Joseph's Healthcare has declared an outbreak at its Charlton campus, where 11 patients have hospital-associated C. difficile.
C. difficile, which causes diarrhea, is one of the most common infections in hospitals and long-term care facilities.