YA novel - Gang of Five?

The book was likely published in the early 2000 no later than 2006. It revolves around a group of 3 or 4 teens who band together in school and form a strong friendship. They call themselves the Gang of 4 (or maybe 5) even though there are fewer of them in the group than the title of their group. I think it’s about bullying, friendship, standing up in the face of adversity, etc.