As with previous Grand Theft Auto games, Grand Theft Auto IV features police missions where you can impersonate officers to take down criminals.

Grand Theft Auto IV is an action-adventure third-person shooter video game developed by Rockstar North for PlayStation 3, Xbox 360 and PC. The title was released on April 29, 2008 and it was published by Rockstar Games and Take-Two Interactive. Grand Theft Auto IV is the sixth episode in the GTA series. http://www.rockstargames.com/IV/

The police missions count toward 100 percent completion of the game. There are two types of cop missions in GTA IV: Vigilante missions and Most Wanted missions. Both become available after you complete the Crime and Punishment mission and can be accessed from any police vehicle computer.IGN: Grand Theft Auto IV Game Guide: Side Missions

On the police computer, select the View Current Crimes option. A map displays all the crimes being committed in your area. Select a crime and an automatic GPS route will be generated. You can then drive to the location on your map and take out the criminals.

The vigilante missions are infinite, but only twenty of them need to be completed in order to earn 2.5 percent of 100 percent completion of the game.IGN: Grand Theft Auto IV Game Guide: Side Missions

On the police computer, select Most Wanted to view a list of the most wanted criminals. There are 10 different most wanted criminals on each island—Algonquin, Dukes/Broker/Bohan and Alderney—totaling 30 in all of Liberty City. When all 30 most wanted criminals are eliminated, it adds 2.5 percent toward 100 percent completion of the game.IGN: Grand Theft Auto IV Game Guide: Side Missions