The call came in at 3:18 p.m. Monday for an address at 360 Morningside Ave., which is located on the east side of Morningside Park.
Toronto Fire Services say all four people are conscious and breathing.
Three ambulances are on the scene.
Paramedics later reported that lightning had subsequently struck a tree beside one of the responding ambulances, which left it "out of commission" as a result.
The city is currently under a severe thunderstorm warning and lightning and heavy downpours are being reported throughout the city.