Rockstar Games has released patches today for both the Xbox 360 and PlayStation 3 versions of GTA IV, which address a number of multiplayer connectivity issues including freezes during certain modes, continuation when players leave matches, and bugs within races and race lobbies.
Here is the full list of issues which have been addressed:
Fixed an issue where players were unable to enter or exit vehicles in GTA Race.
Fixed an issue where scripted vehicles were causing games to unexpectedly crash.
Improved reliability and frequency of PS3™ online leaderboard updates.
Fixed a rare hang some players encountered when moving from a Race or GTA Race lobby into gameplay.
Fixed a hang that would occur when players were switching teams during the countdown in the lobby.
Fixed an issue where players were able to get ridiculously high scores from Cops 'n Crooks matches.
Changed GTA Race and Race so that the last player remaining always receives a reward of $250.
Fixed an issue where spectating players would prevent the game from finishing.
Fixed an issue where players were unable to spectate in Round 2 of Cops 'n Crooks games.
Changed countdown timer in lobby so that it no longer resets when new players join a game.
Changed the kick option so that kicked players are no longer brought back to single player.
Multiplayer patch released by R*
Multiplayer - Race Modes

RACE
1-16 Players
Race to the finish through each checkpoint in order: If you take a wrong turn or mess up in some way, you can respawn your vehicle on the last checkpoint you passed. There is a large selection of races for cars, boats, and helicopters - you can choose laps and vehicle class/type. This mode has two sub-modes: Free Race and Cannonball Run.
Free Race are point-to-point races. First one to reach the end point wins.
Cannonball Run are races where you have to traverse several checkpoints any way you can.
GTA RACE
1-16 Players
Similar to the orthodox Race, but with the added element of combat. Race and fight your way through the checkpoints in order and then hit the finish, but this time you can also pick up the weapons spawned en route to help take out your opponents. You can also exit the vehicle during the road races.
Grand Theft Auto IV Multiplayer

Ever since Grand Theft Auto 3 was released to the world, gamers have been scrambling to jump into Rockstar's virtual reality of violence and corruption. While a handful of modifications have done a superb job of bringing online crime to the masses, there's always been a lacking presence of multiplayer in the official games themselves.
Until now.
With Grand Theft Auto IV, Rockstar has finally delivered a fully fleshed-out multiplayer experience. This is what we know so far:
Total of 15 game modes, each supporting different numbers of teams/players. The maximum number of players is 16.
Configuration of each mode can be adjusted by the host. Vehicles types, volume of traffic, number of pedestrians, police presence, weapons and weather can all be controlled.
Characters are completely customisable. You can essentially create your own unique online character by adjusting their hair, sex, race, clothing etc.