Mission: Impossible IV is currently in pre-production.
In June of 2009, it was announced that Tom Cruise would be returning as Ethan Hunt in the new movie and that both J.J. Abrams and Tom Cruise would serve as producers. It is not yet known whether Abrams will direct, as he did for Mission: Impossible III.
In August of 2009, it was announced that Josh Appelbaum and André Nemec would be writing the script, based off a story written by J.J. Abrams.
The release date for the film is December 16, 2011.
Rumors
Rumors at one point had arisen that Brad Pitt may become the star of the movie, although it was currently unknown at the time if Tom Cruise would or would not be chosen by Paramount (given their strenuous relations).