We've experienced a similar situation with our 4 year old son. When he was 3, we would let him watch some shows where the kids had bad attitudes or were generally annoying. I didn't think too much of it until my sister said something about linking her kids' behavior to shows on TV they were allowed to watch (I love my neice and nephew, but they have SERIOUS attitudes). I started thinking about it and monitoring his shows a LOT closer. We have it narrowed down so he is only allowed to watch shows that have been pre-approved, like Dora, Deigo, Blues Clues, Wonder Pets, Backyardigans, Handy Manny and any of the cartoons on PBS or TLC. We cut out some other shows that he used to watch. When he asked to watch them, I just told him that I didn't like the way the kids on the show act, and it wasn't a show I wanted him to watch. He accepted that right away. In fact, if one of those shows comes on and I'm not in the room, he turns the TV off. I also limit my kids' viewing to certain hours (when they first get up, and when their rest time is over), and for only about 2 hours a day. I also scroll through the viewer guide on the TV to see what's on, and give him the options of what they can watch. That way, if there is a show on that he would want to watch but I don't want him watching, he doesn't know it.
Another great way to avoid shows I don't want them watching is to borrow videos from the library. I let my kids get 2 or 3 videos at a time, and they will watch those over their favorite shows most of the time.
Good luck!!