GTA 4 cheats can be activated by entering one of these numbers into your cellphone, like you are calling that number.
Each time you activate some cheat codes, "Cheat Activated" message will appear.
Be aware that every attempt of cheating will be archived by game engine, and can be seen at the game statistics.
Also, cheating may affect some missions, as well as achievements.
482-555-0100 - Give health, armor, and ammo
486-555-0150 - Get different weapons
486-555-0100 - Get weapons
267-555-0150 - Raise wanted level
267-555-0100 - Remove wanted level
362-555-0100 - Give armour
948-555-0100 - Current song info
468-555-0100 - Change weather
227-555-0175 - Get a Comet
625-555-0150 - Get a Sanchez
227-555-0142 - Get a Cognoscenti
227-555-0168 - Get a SuperGT
938-555-0100 - Get a Jetmax
625-555-0100 - Get an NRG-900
227-555-0147 - Get a Turismo
359-555-0100 - Get an Annihiliator
227-555-0100 - Get an FIB Buffalo