death is what life is all about!
Not for a 4 year old IMHO. He can understand about schar and onesh, even Olam Haba, without knowing that one day ppl die.
I admit my son is the sensitive type, and if he would be more macho, I might have toldl him about death.
Maybe that's the whole difference between our approaches. My son would be shaken up and disturbed to hear about death. In a year or less he may toughen up enough to hear about it.
Like Shlomo Hamelech said - Chanoch Lenaar al pi darko.