SG-1 hasnt ended yet here, infact it just played an episode on friday, 13 or 14. The series might be over but the straight to tv movies CONTINUE exactly where SG-1 left off. And in the fourth season she wouldn't be the main character. They wouldn't take someone from SG-1 and instantly make them the main character without killing off someone on the cast with the same skills. And furthermore, its way to early to tell. you might see glimpses or previews, but if you check gateworld, they only have the description of one episode, so we don't know who plays the main part in those. My guess is that carter will not be in every episode, maybe the first couple, but not all. I think it might be possible in the 5th season for her to be worked in more.