In the past 18 months, we've seen a lot of excellent fourth sequels released. Look at the list:
Call of Duty 4: Modern Warfare, Nov. 2007, 9.0
Grand Theft Auto IV, Apr. 2008, 10
Metal Gear Solid 4, June 2008, 10
Street Fighter IV, Feb. 2009, 9.0
People like to say that the third time's the charm, but maybe the fourth time works out pretty well too.