The video game system version is rated M, but the computer version of this game is rated T. My boys let me know about this. The 4 year old can't be around when the older boys play.
In this game you go around shooting aliens. Maybe the 4 year old has big brothers that he has seen playing the game. No 4 year old would really ask for it, unless they had someone influencing them.
So if you are going to have playdates with this kid, the way you can get around it is say something like "I only like my son to play E rated games if you happen to have a video system."
Many years ago I had this problem with my oldest. There was a kid in the neighborhood who liked to watch Power Rangers. My son would watch it and then karate kick and punch us, so I told him he couldn't watch it. I had to talk to the kid's parents and the nanny that watched them during the day about it. They didn't agree with me and ultimately I couldn't let my son go over there, but I welcomed having the kid at my house.
I'm sorry you have to go through this with a 3 year old. There is lots of parental peer pressure out there. Stand up for what you think is right!