Alberta law is pretty slack on Carseats.
http://carsafetyandkids.ca/user_uplo...t_Jan_2013.pdf
Basically, once they hit 6,
OR 40 pounds, they no longer need to be in a car seat of any kind.
My little guy is just over 4, and weighs over 40 pounds. I still have him in a forward facing full back booster, don't see the need to rush him out of it, way I see it, is so long as he's comfortable, the safer the better.