I think my son took 1 aspirin but he won't tell me cause he has austim ?

Reye's Syndrome: The use of aspirin in children is generally not recommended due to the remote risk of Reye's syndrome, a condition of swelling of the liver and brain during a viral infection. This problem is usually heralded by excessive vomiting. This condition would be highly unlikely after ingestion on a single aspirin, but monitor closely. Otherwise, this ingestion should pose no significant health risks.

No big issue: For years we used aspirin for days at a time for pain and fever reduction in kids, but quit in the 80's after it was associated with liver failure after chickenpox or flu. A single dose in your 4 year old during his usual life is unlikely to matter at all. You might use the experience to increase safeguards against self medication or toxic ingestion.

No harm: At 5 years of age, one aspirin should not be harmful. However, it is an indication that he has access to your medications. For safety sake all medications should be out of his reach and in child proof containers.