Delayed speech. A 4 year old with autism will have had delayed speech and language. They will also play alone more and not want to interact with others as much as a non-autistic 4 year old will. There are always grey areas so if you think your 4 year old is not developing normally or talking as the should, see your pediatrician as soon as possible. Early intervention is very helpful at that age for your child.
Social interaction. Autism is a spectrum of symptoms. Children with autism/pervasive development disorder/asperger's syndrome tend to have difficulty interacting with others, especially other children. Mild cases can be difficult to diagnose. Sometimes, parents say their child isjust "different". They tend to have poor eye contact. Some children find comfort in doing repetitive motions like rocking or banging.
More. . . In addition to dr. Kaplan's answer, understand that many PDD kids are unusual socially. They may be awkward, repetitive, often unable to understand "w..." questions such as where are you, when did you...?, who was that? Time questions are also difficult for them such as "how old are you?" PDD kids use jargon, or trivial phrases verbatim (echolalia). Pdd kids repeat tv commercials, movie lines.