You can gather up to 16 friends (or enemies) and go bombing around liberty city looking to kill each other. Another addition is online multiplayer, A first for the GTA series. but you can also play darts, pool, and go bowling with your girlfriend.
You can just hotwire a 69 Charger (known in the game as a dukes) or steal a late-80's, Early 90's era Camaro (known as a Ruiner) and just go bombing through the city running people over, getting in trouble with the law, and hiring Sleazy women for their dirty services (you get to pick which one you want, and the game actually shows it happening this time! Yay!) Sure, you could do that before. as Always, you don't even need to follow the story to have fun.
Remember what I wrote before how Rockstar could push their franchise twice as hard as before? Well, GTA IV goes beyond twice. Rockstar finished working on what is now a classic, Grand Theft Auto IV. Pretty soon, Rockstar improved on their franchise greatly with Vice city in 2002, and then San Andreas in 2004, proving that they could push the capabilities of their game twice as far as they'd done before. back then 3-D free-roaming had not been accomplished like Rockstar and take-two interactive had done it. GTA III on the 6th generation of consoles was a breakthrough in Gaming in 2001.