You have named all four of the main characters. They call themselves the Gang of Five even though there are only four of them. They figure that is a good name since there is likely another social outcast like them that will need to join the group someday.
There are other characters in the book that make frequent appearances, but I would not call them main characters. The first would be Mr. Kiley. He is the school principal. Another is Mr. Kellerman. He is Bobby's boss at the tie store. Ms. Wyman is Addie's homeroom teacher, and it is in that class that Addie refuses to say the pledge of allegiance. Kevin Hennessey doubles as a popular type kid who also is a bully to the Gang of Five. Dushawn Carter also functions in that capacity.
They call themselves the Gang of Five, but in reality there are only 4 of them. You have named them all.