I just asked him the same question he asked me 5 years ago. His answer? "Music Player 3 lor"
He's a big boy now!
He is only 4 years old, he does not understand how bad a train crash is. Perhaps he does not even know what is the meaning of death.
A four year old child might be limited by their choice of words when posing the question. There is no malicious intend on your boy's question. You did good in explaning to him how death impacts on those survived.
My son still remember what is being explained for eg he is able to describe & draw evaporation/condensation process, some facts on global warming, the Green Peace movement, cold/warm blooded animals, what is ERP, why it's needed & why it's not working. Usually I will explain when I have the time which means he got to wait for at least a week but he will remind me.