In my opinion, Harry Potter and the Goblet of Fire's story was not very predictable. I anticipated that at some point, Harry would have an encounter with Voldemort (or one of his supporters) and emerge alive, but I could never have seen some things coming, including Harry's name being put into the Goblet of Fire (or the Triwizard Tournament coming to Hogwarts in the first place), other schools bringing students to Hogwarts, and so on. One thing that particularly threw me was when Voldemort came back, killed Cedric Diggory, and almost killed Harry. I didn't think Voldemort was coming back until at least the fifth book(although I knew he was coming back eventually)--it was too soon! It seemed like, by the end, there were huge storm clouds looming over Harry and all of his friends, because Voldemort was back, and the Minister of Magic, Cornelius Fudge, refused to believe it, which meant that nobody would know about it.
In conclusion, Harry Potter and the Goblet of Fire has a darker story than the other books in the series, it seems, and I did not see many things coming. So, therefore, I must say that it was not predictable at all.