The John Williams Fan Network is citing two "very reliable" sources in their report that the composer may not be able to score Harry Potter and the Goblet of Fire due to scheduling conflicts:
According to one of the sources (someone very close to the composer), Williams might write the score for Memoirs of a Geisha. Both Harry Potter 4 and Geisha are scheduled for release in Fall, 2005.
The film rights to Memoirs belong to director Steven Spielberg, who often collaborates with Williams. If that's Spielberg's next project, Williams may be too busy for Potter, although "things might change in the coming months," reports JWFan.net. Sad news, considering Goblet of Fire is reportedly going to be the most expensive movie ever made.