Finally the script has been finalized. Shooting begins early 2007 and the movie is due out in 2008.

The Story of Jurassic Park IV

65 million years ago, a terrible disease plauged the Earth which brought about the extinction of the dinosaurs. In the eartly eighties, when InGen cloned the dinosaurs, they also brought this disease back with them and it has been present ever before since Jurassic Park (thus explaining the sick Triceratops and Brachiosaurus). Now this disease has become fully active in the dinosaurs and they're all getting very ill. When dinosaur carcasses start washing up on the shores of Costa Rica, a team (including Alan Grant and Lex) is called out to find out what's going on. They head to the islands to study the dinosaurs and find out about the disease, known as DX. They also discover the disease is transferable to humans and could possibly bring on our own extinction.
That's the finalized script for Jurassic Park 4. The star of the movie is going to be Lex (Ariana Richards) the little girl from the first movie. Sam Neil has also been confirmed and possibly Jeff Goldblum from the second movie as well. I can't wait to see it, I'm hoping that they'll include all of the characters from the past movies in it as well somehow.