With a 4.5 year old I think its good to ask questions. You want to figure out what he was thinking, so that you can offer the appropriate re-direction. If he really, sincerely wasn't thinking (and that could tell a while to get to, since "I don't know" is always the first answer), it may indicate that he was feeling overwhelmed or nearing a meltdown, in which case avoiding the trigger situation will be more effective than consequences.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).