Look for season 5 on APTN next Sept. This is a just a guess, but season 4 appeared on APTN exactly 2 years after its release on Netflix. Perhaps Netflix had a Canadian exclusive for that time or perhaps APTN didn't want to pay the higher price that that a season gets 1 year after its release. I believe the price is lower 2 years after its release.
APTN is airing season 2 next Friday.