David noticed in this month's issue of Scottish soundtrack magazine "Movie Music Now" (issue #11), they had an interview with composer John Williams. It was mainly focusing on his long career and such, but towards the end of the interview, he mentioned Jurassic Park IV. Here is the extract:

Can you give us a rundown of the film scores you have upcoming?

WILLIAMS: Well, I'm certainly busy as ever, which I enjoy. Harry Potter and the Prisoner of Azkaban is definitely in my future. Star Wars Episode 3 will follow that, which I'm very much looking forward to doing. It's like the completion of an epic journey. Then I'm back with Steven Spielberg for the fourth Indiana Jones, an exciting project that seems to have attracted incredible interest. Steven has also expressed his desire for me to score the next Jurassic Park film, which he is developing presently. It's a genre I would just adore to revisit, so I'll do my best to be available for it.