The 4th entry will be titled Mission:Impossible Ghost Protocol. The name was announced at a press conference in Dubai attended by Tom Cruise, director Brad Bird and co-stars Jeremy Renner and Paula Patton. It’s in production now and is scheduled for release in December 2011. Besides the Dubai location, other shooting spots include Moscow and Prague.
What does the title mean? In CIA parlance, the term "ghost" can refer to some of the suspected terrorists held at Guantamo Bay and other secret bases around the world. Could the plot involve Ethan Hunt and his team being assigned to hunt down an escaped "ghost?" Just speculating.