When you originally check in a bag, it's 4 hours. However, you've already checked the bag in Detroit and it should be tagged to your final destination. After you go through Canada Customs at Pearson, just drop it off at the connecting baggage area.
If the bag is not tagged all the way to Vancouver, you will have to to to the airline counter to check it in again, and the 4 hr limit will probably kick in again.