York Regional Police say the 20-year-old woman, whose name hasn't been given, was pulled from the swimming pool unresponsive, and remains in hospital in life-threatening condition.
Insp. Russ Bellman said the girl called 911 just before 5 p.m., after her babysitter accidentally fell into the deep end of the in-ground swimming pool in Newmarket, Ont.
He said paramedics, police and fire services responded to the call, and conducted CPR on the woman before taking her to hospital.
Investigators say the incident isn't considered suspicious, and the child was not injured.
``She's a pretty sharp four-year-old,'' Bellman said.
``It just demonstrates how important it is to teach your kids _ even at a young age _ recognizing an emergency, knowing when and when not to call 911. And in this case obviously she recognized the emergency and knew enough to call 911.''