Found it on Wizard City:


Warner. bros has decided that, despite the size of the GoF book they are going to make it as one single movie. Since the appointing of their new Director for the film, Mike Newell things are beginning to come together.

Afraid of alienating die-hard Muggles by cutting scenes, the studio and scribe initially had talked about breaking the [Goblet of Fire] book into two separate films to be released in the same calendar year, à la The Matrix Reloaded and Revolutions, but Warners has now opted for a single movie.

It also seems that some serious thought is going into the main trio, Dan, Emma and Rupert. Although we've been told that they've signed up for the film there is now time for speculation. Warner bros. are considering whether the aging actors should be kept or whther to re-cast their positions.

A rep for Warners said the studio will likely bring back the franchise's aging kid stars -- Daniel Radcliffe, Emma Watson and Rupert Grint -- for the fourth adventure, but it's possible filmmakers may decide to recast. A separate announcement regarding those roles will be made soon.