Get some parenting books to learn how to effectively discipline your child. Be consistent. Try time outs. Don't give in. Take his favorite toy away for 5 minutes.
You aren't losing your sweet boy! I promise you. He's still right there and you need to tell him everyday, a hundred times a day that he's your little sweet boy. Tell him you love him. Teach him kindness by demonstrating it...to all.
Play with him. Get silly with him. Remember, he's only 4!
Dying? I promise you he does not know what that word means. He learned that word somewhere and he is echoing it. Has there been a death in the family? Where DID he learn that word. We never use that word, or rarely. We do a little now that my son is older - like with a bug. But they still do not understand death's permanence.
Do boys hit, kick and pull hair? Yes. I have two of them. When my sons hit each other, they apologize immediately and then face the music: A time out. Not negotiable.
Hug and cuddle both your kids - at the same time.
Anyway, NEVER, never give up. Be strong for your son. Also, reach out for help and support.