@Ingrid531 The overall rating is not an average but a separate category. So the guest has specifically selected a 4 star rating. You could message the guest and say something like "Thank you again for staying and for your review. I see that you have given your stay an overall rating of a 4 star. May I ask what it was about the stay that did not earn a 5 star review. I am always trying to improve so any feedback would be greatly appreciative."
If the guest responds that could give you some clarity.