Police say the victims include two women and two children who were fatally injured when flames broke out Tuesday night at a home in a north-end neighbourhood.
The baby who was sent for treatment is in critical condition.
Media reports have said the adults were a mother and grandmother and the children are girls, including two under the age of 10.
It's believed the fire started in the basement.
The city's arson strike force is investigating, but police say the blaze does not appear to be suspicious.