She does it specifically to annoy me, she was also pounding on my head where I hurt it in the car when I was cleaning the car, so I could earn a new CPU.
She's four, dear. Maybe she just wants attention or to play. Patience is sometimes difficult with children, but necessary. Just try to mind that it is your sister. As much as she may bother you, that doesn't change. Appreciate you have one for now. Try to build a relationship with her. Many long for a sister, but never had one. And some have one but are drawn so far apart for different reasons.
The main thing that children and teens don't understand is your parents have feelings and lives. They don't babysit you, they rear you until you're ready to leave the house and live on your own. People who rebel and try to prove they are right end up having too many social flaws that do NOT work out well in the real world.
Life isn't like school, kids.
Many kids rebel to individualise themselves. Most do in someway or another. But many children do fail to realise their parents have lives outside of the household, yes. And especially in the teen years, their monkeysphere [/funreference] may draw to a point where they see their parents as they do teachers - just 2D authority figures. And that's where conflict may arise. :s
But of course, if all else fails, you could try voodoo mind tricks on the girl, so she will cease without knowing you are the one making her do so. How? I'm not sure. But then you have to bring something to the table, too. :o