VFXWorld has extensive coverage on the VFX work in Harry Potter and the Goblet of Fire which covers the work by ILM, Double Negative, Framestore CFC, Cinesite, Buf Compagnie and The Moving Picture Company.
You can read them here:
VFXBlog also has several interviews with supervisors from Rising Sun Pictures, Double Negative, Anima Logic and The Orphanage, plus links to other online coverage, about the work on the film:
Rising Sun Pictures on 'Goblet of Fire'
Double Negative's Mark Michaels on 'Goblet of Fire'
Animal Logic's Kirsty Millar on 'Goblet of Fire'
The Orphanage's Jonathan Rothbart on 'Goblet of Fire'
Framestore CFC's Trial by Water in 'Goblet of Fire'
Harry Potter "La Coupe de Feu"
SyFlex in Harry Potter and the Goblet of Fire
MPC showcases 'Goblet' visual effects