In GTA IV cheats are activated via using the mobile phone. Try entering any of the following codes after Roman has given you his old mobile phone. This happens pretty early in the game so you don't have to wait long before you can enter the cheat codes. Once you have the phone you can enter them as often as you like.
Okay, first get out your mobile phone and press 'Up' to raise the phone higher and reveal the complete keypad. Then use your controller to select the set of numbers according to which cheat you want to use, after you have entered the code the cursor moves to the dial button, just press 'Up' again and if successful you will see a message 'Cheat Activated' on the screen. When a code has been entered correctly, a new 'Cheats' menu option becomes available on your phone under 'Options' which allows you to access the cheats without having to enter the phone number again.
Cheat Code Entry
Full Health and Ammunition:
Reset Wanted Level:
Wanted level Up One:
Change Weather and Brightness:
Spawn Annihilator Cop Helicopter:
Spawn FBI Buffalo:
Added by: Team SuperCheats Apr 29th 2008, ID#540