Assuming that the three seasons were the three years of them being there, and the movie having (somehow) their fourth and final year, I don't see the outcome of a fourth season. But then again, it never states in the movie that they are in a different year of school rather than the same of season three. So, I'm completely confused on the topic.
If the fourth season is made than that could be the last, or if they make another season after that it could be the last, etc. But for right now when your talking about 'seasons' and not the Touchstone of Ra, the third season is the last one that has been made.