It's one thing to do stupid things and another to be malicious and intentionally trying to hurt people...which was something Brooke did in season 4. I don't care how much you think you're hurting, nothing gives you the right to treat people that way.
Then, there was the whole stealing the test SL, lying to her friend and getting her fired, slutting up again and sleeping with her teacher (I'm not from the States, but wasn't principal Turner required by law to report Nick Chavez to the police for having an inappropriate relationship with his student?), making fun of Lucas' mixtape (which made her seem completely shallow), the dead mom jokes...there was almost nothing likable about her that season as far as I am concerned.
But even if I could accept and find it believable that she completely changed her personality, is was 100% unrealistic the way she became friends with Peyton again that quickly after everything they did to each other.