I've not experienced this from the parent side, but I was nearly phobic about insects as a kid. My mom walked into a room and found me crying in a corner because there was an ant in the room. I still have my moments with insects, but what helped me was talking about what I was afraid of, learning interesting insect facts, and (this might sound weird) learning how to empathize with insects. What I mean by empathize was that I came to understand that they were going about their lives just like I was going about mine, that they didn't choose to be insects any more than I chose to be a human, and that they were busy doing something else, not looking for a way to get me. That all might be a bit much for a 4 year old, but that's my personal experience.