From IV:MP Wiki
IV: Multiplayer (IV:MP) is a modification for Grand Theft Auto: IV (GTA:IV) founded in late 2009. It turns GTA:IV into an alternative multiplayer game. You can play over the internet (or LAN). You will need the original GTA:IV PC game in order to play IV:MP.
In IV:MP you are able to do almost anything that you can do in single player GTA:IV. IV:MP features the possibility to create your own scripted modes. There are many game modes that feature a series of different and fun things to do. Many of which are not possible in single player.
IV:MP is not in any way affiliated with VC:MP or SA:MP.
IV: Multiplayer (IV:MP) is in no way affiliated with Rockstar Games, Rockstar North or any of Rockstar's subcompanies.