You still can kill police, but it's wiser to avoid these shootouts because if you're arrested, you'll lose all your money and weapons and fail the mission. Though they'll try to jump out of the way, you also can drive into pedestrians.
Players can again proposition prostitutes to get a "health boost." Exotic pole dancers are in the game — but there is no nudity, nor is there a hidden sex mini-game, à la "Hot Coffee," which landed the game publisher in hot water in the past.
You also can drink and drive in this game, and if you choose to go out partying with friends, at the end of the night you can walk home, hail a cab or drive; if you drive, your vision is blurred, making it harder to control the vehicle.
Finally, GTA IV contains plenty of foul language.
In short, heed the "Mature" rating on the box. It's not appropriate for players under 17.