Sibling fight

So my oldest is 4 and my second is 1 years old. My oldest son always hitting his sister. No matter what. If she just come close to him he will hit her to go away. He definitely hates his sister. But at the same time she doesn't know anything. Her wanted to tap him or etc he thinks she hit him. And when she wants to watch his tablet she lean her head next to him but he hit her. I told him that she's still learning and that he's not being very nice. How do I go about this? It's hard to tell him to quit hitting..