Lucasfilm says Indiana Jones IV sequels are rumours
So Lucasfilm are coming out and saying that the talk of multiple sequels after this Indiana Jones is merely a rumour. Now that's not a complete denial, but it is telling everyone to calm down a little.
Still, as I said before, I think that it could well be in the relevant contracts but it's still just a standard place holder for the actors. I'm sure that the intent is to close down this series with Harrison Ford, but I do imagine that they'll head for spin offs if the film does well.
The quote comes from the Lucasfilm Director of Publicity John Singh through MTV Movies Blog:
"That’s purely a rumor...Idle chatter among Internet fans."
It is interesting that they aren't actually saying no, and that's what makes me think that they might well be thinking of a spin off or a restart with Shia LaBeouf as the young Indiana Jones.