The guys over at the GTAForums have been busy translating the Spanish issue of PSM magazine that includes a GTA IV feature.
There will be support for up to 16 player multiplayer and 15 online game modes including: Hangmans NOOSE, Car Jack City, Bomb da Base, Bomb da Base II, Mafia Work, Team Mafia Work, Deathmatch, Team Deathmatch, Turf War, Cops N Crooks, Race, GTA Race, Deal Breaker, Free Roam and one more unknown mode.
Other details include that you can drive a car or a helicopter with three friends and the host of the game decides rules such as the flow of traffic, the intervention of police and the weapons. To quote:
Car Jack City
Bomb da Base
Bomb da Base II
Team Mafia Work
Cops N Crooks
and i believe there is one more.. its says theres 15 game modes total
15 multiplayer game modes, they play seven.
_ The multiplayer modes are played in the same map than single mode.
_ You can drive a car or an helicopter with three friends
I will continue translating... Well I'm spanish so here go some bits:
The host of the game makes the rules, in "Race" and GTA Race" he decides the vehicles and laps, in Deathmatch and Team Deathmatch he decides the number of peds, the flow of traffic, the intervention of police and the weapons. We tried a match, all of us armed with rocket launchers, half of city lighting up. (or something like that)
EDIT: Mafia and Team Mafia work, you will play as one of Kenny Petrovics mafia henchmen, competing to complete hits ordered by the boss over your phone before your rivals. In "cops n crooks" A group of players will play as police, other as criminal, the police must make sure they don't escape alive, they can also see the criminals location ont heir GPS while criminals have no clue as where the police are.
Also in the single player mission "hangmans Noose" there will be co-op, the mission you and your friends will be able to escort a mafia man, while ambushed by the SWAT, but it doesn't end there, carjack, turfwar, bomb da base II, free mode, the 100 hours of single player are nothing compared to the 10 years of multiplayer.
EDIT: Also, there are a total of 15 Multi Player modes, all accessed through your cellphone. You will be able to CREATE YOUR CHARACTER FOR ONLINE, change his race, sex, skin color, hair, clothing. 16 players online.