Why?

Why is it that the 4 year old has to have EVERY freakin' toy that his sister wants to play with? Why does he have to try and take EVERY toy she plays with away? And then when I take it back from him he has a complete breakdown? Is it an age thing? Is it a sibling thing? Is it jealousy? How can I stop it?

Now he is hitting the wall b/c he is mad. I just sent him back to time out b/c of it.