My son keeps hitting my daughter

My son is 1 1/2 years older than my daughter. He is often patient when she plays with him (they are 6 and 4). But he will frequently hit her. No matter what I do to handle this, he keeps doing it. I've used timeouts, sent him to his room, taken privileges away, talked to him, everything short of hitting him. I believe hitting him would just be hypocritical when I'm trying to send the message that you don't hit people.
What do I do to get him to stop?

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