ComingSoon.net has called the Yule Ball scene in Harry Potter and the Goblet of Fire the movie's centerpiece, in their set visit preview.
The fourth film also marks the first on-screen appearance of Harry's arch-nemesis, the dark wizard Voldemort, as played by Ray Fiennes [sic], although the way he'll show up in the movie is the biggest secret at Leavesden these days.
Production designer Stuart Craig, who crafted the look and feel of all the Potter films to date, designed an "enormous" graveyard, and creature effects supervisor Nick Dudman built a full-size animatronic dragon.
ComingSoon.net also says that Miranda Richardson, as tabloid reporter Rita Skeeter, will provide much of the fourth film's humor.
Update: MuggleNet has posted a call sheet that provides a few details about the Yule Ball.