Terminator: Salvation: The Future Begins, otherwise known as Terminator 4, is going to happen. Now that Warner Bros. has acquired North American distribution rights to the film, which is supposed to be the first in a trilogy, you can bet that the film has been fast-tracked and is on the way to getting done.
The script is already written, by John Brancato and Michael Ferris, who wrote T3, and McG has been rumored to be directing the movie. There's even an outside shot that Arnold will make yet another cameo appearance, but that's totally unconfirmed.
Dark Horizons.com: Warners Set For "Terminator Salvation"