From an interview at theindyexpereince.com:

IESB.net: Is Indy 4 still going to be made??
Ford: “No doubt about it.”

IESB.net: Could a younger actor play Indiana Jones in Indy 4?
Ford: “No problem, but Indy IV is mine.” Ford also added that he wouldn’t mind having a younger actor play Indy in future films, which is similar to what was done with the James Bond films after Sean Connery left.

IESB.net: Will you be doing your own stunts for Indiana Jones 4?
Ford: “At least some of them.”