Your guest was either ignorant, spiteful, careless or someone who “doesn’t believe in 5* ratings.”
Airbnb’s review instructions are not at all helpful.
It has happened to nearly everyone. You could try to educate and/or embarrass them with a sorrowful personal note but it would probably be a waste of time.
@Leanne-and-Darrin0 Yes this does happen occasionally. It could be that they felt every category deserved 5 stars, but there may have been something else that didn't fit a category that they felt was lacking. You'll never know unless you ask them. Keep in mind that many guests are clueless as to how devastating a 4 is to hosts. They are blissfully ignorant. Airbnb doesn't help that. On purpose.
@Leanne-and-Darrin0 The overall rating is not an average of the separate category ratings. It is a rating unto itself, and the one Airbnb uses as the rating for your listing.
@Leanne-and-Darrin0 I think you were lucky to only get downgraded to 4 stars. Based on your (and other) review of this guest they are clearly not suited to Airbnb.