The newest entry in the Terminator series, Terminator Salvation: The Future Begins, will open on May 22nd, 2009, making it one of two big name films to open over Memorial Day weekend. The other will be the Ben Stiller sequel to A Night At The Museum, which should appeal to an entirely different audience altogether.
T4, meanwhile, will star The Dark Knight's Batman, Christian Bale, and will begin filming on May 5th of this year. Not much is known about the plot, other than that it will be the beginning of a three-picture arc that will provide us with all of the Terminator action we can handle for the next few years.
Variety.com: WB's 'Terminator' to open May 2009