What Leah's child is asking is not when he'll die, but what happens to his story. My son asked about the same age when we were showing him family photos "Where was he?" We explained he hadn't been born. This was the upsetting question for him. That he may have not existed at some time. Death means there may be a time when he doesn't exist in the future.

Since all we know is what we know, it the genuine Cartesian sense. Leah's child is obvious quite intelligent. But what a 4 year old can understand is limited. Invoking science is not likely to calm his fears. Death isn't about mortality. My son didn't like The Story of Babar, not because Babar's mother is shot and killed by a hunter, but because the old elephant king ate a bad mushroom and died. 

The great thing about children is they think in an original way. We tend to be smug. But kids know something odd is going on. Life isn't fair. Death comes in mysterious ways. And a week is an eternity. We forget this. We need to reconnect.