Sit next to him, hold a knife next to his stomach so that no one will see it. I would take all of his money, his identification cards, so I know where he lives etc. and I can pay him a visit whenever I want. I'll ask his little brother where he goes to school and act as a friend of his big brother. Now with information like that I could be really dangerous, life threatening. He would hack for me, he would pay me cash weekly, or I will destroy him. Police? Let him proof it. If they keep an eye on me or on him, I'll just don't come in contact for some months, then one day, I would throw a molotov cocktail through his window and stand there until only he sees me. I leave the crimescene, again, proof it. Ohh what a slave he would be.
Nah, I'm kidding. I would probably bury a chair in his neck and kick him untill his face is all covered in blood. Forget about the 4 year old kid, when he groes up, he won't have any memories anyway...