I bought GTA IV for PC in December of 2012 mainly to play LCPDFR, a popular modification that allows players to be a police officer. Just recently, I played Grand Theft Auto IV: The Ballad of Gay Tony as well as the main GTA IV, and they are both very obscene. Even my uncle (when he and I were playing and taking turns) said the language was just outrageous. My uncle does not know that I am educated about sex and he doesn't feel right when characters make sexual references. Which they do, A LOT, and that is the reason this game is slightly more obscene than the main GTA IV. Let's just go over some points here:
Violence: There is a lot of violence in this game. However, I would not be concerned about violence, as there is not too much gore, and come on, with all that's going on in the world, you have to educate your child about violence and the way things work now. I, personally, am very against guns, and take no interest in them, however this game happens to use them. The graphics are not as realistic as some of the newer games, but some missions do involve a lot of gore, especially one mission in the main GTA IV, where the main character (Niko Bellic - a Russian who has come to Liberty City to live with his cousin) has to kill another character. When Niko Bellic pulls the trigger, the character is shot in the head, and blood does come squirting out of his eye, but if your child is mature enough, they should know that killing is a serious offense to religion and the law.
Drugs: So far, The Ballad of Gay Tony isn't too bad with drug references. I'm on one of the missions slightly into the game and it doesn't look like there are many drug references in this iteration of Grand Theft Auto IV, and that's always a good sign, but again, you do need to educate your child on these things in life. If you are playing this, you are probably around 12-13+, so eventually you will have to be educated on those types of things.
Language: This is AN IMMENSE issue with The Ballad of Gay Tony, I mean, it even says a slightly inappropriate word in the title! Of course, inappropriate for younger children. Characters in this game act like the only words they know are obscene ones. They also make lots of sexual references when speaking. For example, there is a mission where you need to take someone and his girlfriend back to where they live. Let's say the guy's name is Dave and the girl's name is Lucy. While they are in the car on the way to the house Lucy says "What the f**k is up with Dave? He bangs any p***y he can find!" or something along the lines of that. I just do not understand what Rockstar North were thinking when developing this game, even my uncle was saying he couldn't stand it! Otherwise, it's usually the normal swearing, but as long as your child doesn't get influenced then that's fine. I have not been influenced by the language in this game at all. I'm just how I was and I've been playing this for a while now.
Not too much other than language. There's more in the main GTA IV. Other than that, not too much to say!