I have a 4 year old daughter, and ever since she was young she flaps her arms when's she's excited or happy. could this be a sign of autism?

Flapping arms: Is one of those characteristic complex repetitive behaviors seen in autism. But children without autism "flap" too. It is not diagnostic Are there any other concerning behaviors? If you have concerns you should talk to your pediatrician and have her evaluated.

Unlikely...: Motor stimulation is common and normal in kids who are excited. Autism is characterized by difficulty with interpersonal relationships, communication, using language and abstract concepts. Does your daughter speak, make eye contact, and engage with you and others? If so, she is not autistic - enjoy the flapping because there is nothing better than a happy and excited kid.

Takes more.: Social give and take problems: not having good eye contact to not being able to join up with peers ideas for play. Prefer to be alone/not seeking to start social interaction. This is central and it is not situational e.g like a social anxiety. Two other key areas: social communication probs and restricted patterns of behavior. Full criteria at: http://www.autismspeaks.org/what-autism/diagnosis