To attain 100% Completion in Grand Theft Auto IV, the player needs to do the following:

• Complete the main story missions (The Cousins Bellic to Out of Commission (Revenge) or A Revenger's Tragedy (Deal)).
• Complete all 10 drug deliveries for Little Jacob.
• Win all 9 races for Brucie Kibbutz at least once.
• Complete all 10 Exotic Exports for Brucie.
• Complete all 9 missions of The Fixers Assassinations.
• Complete all 30 car thefts for Stevie.
• Brucie Kibbutz - Unlock special ability.
• Little Jacob - Unlock special ability.
• Patrick McReary - Unlock special ability.
• Brucie Kibbutz - Do each available activity once.
• Little Jacob - Do each available activity once.
• Patrick McReary - Do each available activity once.
• Beat a computer or friend at Pool.
• Beat a computer or friend at Darts.
• Beat a computer or friend in a full game of Bowling.
• Beat the high score in the QUB3D arcade game.
• Do all random character missions (excluding Jeff, Ivan, Clarence, and Cherise).
• Complete all the Most Wanted missions via police computer.
• Do 20 vigilante missions via police computer using the "Recent Crimes" option.
• Kill all 200 Pigeons.
• Complete all 50 stunt jumps.

Rewards

The reward for 100% completion in GTA IV is the ability to buy an unlimited amount of ammunition, although it still costs the same as before, and the Key To The City Achievement is unlocked. This reward has a bug that is present when the player loads a 100% saved game, which causes all the ammo that was over the original limit to be lost.