DS (nearly five) is doing the same at the moment. He made himself sick with upset a few weeks ago - worrying about him and me dying. He nearly threw up again the other night, talking about this same thing, but we contained it.
I heard once that four-ish is the age where significant fear kicks in. Don't know if there's any truth in it? It would explain DS's recent fear of dying, as well as monsters in the bathroom which I have to shoot before he'll go in there.
I do the same as muffle. We talk about death being normal and natural, like a big sleep, and all being well, a very long time away. We talk about DS being young and healthy, and the things he can do that can help him have a long, happy life. We talk about the circle of life - he gets the whole gardening and composting thing, and sees me get so excited about the lovely compost we make. He likes walking through the village churchyard, and asks me to tell him the names of the people on the grave stones. So intellectually, I think he gets it. It's just that, emotionally, he isn't mature enough to take it in his stride yet.
Lots of listening, gentle discussing and reassurance helps. And time.