Er, Cho was in the 3rd book? I don't remember her (but I read it two years ago). Anyway, I think she is a minor character so they can probably get away with it. Then again, X-Men has done a nice job of setting up (potential) future characters with cameos. No reason why they couldn't do it with Harry Potter.
Incidently, plans for the 4th movie are rolling ahead. The studio is in the process of finding a director. The only concern seems to be that it might be too long so it will be cut in half a la "Kill Bill". Personally, I don't think it would be too hard to squeeze The Goblet of Fire down to a 2 or 2 1/2 hour movie, but I guess they are worried about leaving things out and alienating fans.