There was another situation about six months ago where two German women stayed with me for about a week. They were overwhelming with their praise of the room, saying how they had previously stayed at an AirBNB in New York City which was terrible. I helped them organize their tours of Washington DC and even called museum information lines and translated for them so they would know hours of operations and featured exhibits.
When they left, 4 stars all over the place in the individual categories and 4 stars overall. Then it occurred to me, the last few days of their stay the other room had been occupied by an African American gentlemen. The German women had been using the shared bathroom as if it were their private restroom with hair products everywhere and underwear drying on the shower rod. I asked them to please take personal items out of the bathroom and the one time I observed them passing this black man in the hall, they gave him looks like daggers.
It was enormously clear they downgraded their rating to 4 stars because this gentleman had been present and they clearly didn't like a minority using the bathroom they also used. There was no proof however, so nothing could be done. The situation I described in this thread is a bit different because the man involved actually sent a message saying he wanted to use another bathroom. I will see what AirBNB does; maybe they will take the review down. As you said, one 4 star is not that big of a deal but I have had an unfortunate streak of jerks coming through leaving 4 stars for no obvious reason. Not enough to make much of a difference, but everyone does hurt a little.