It wasn't dirty, @Jane563 ? I understood that cleanliness was the issue, such an issue that knocking off an overall star, not just a cleanliness star, was well deserved.
An honest opinion is fine in the real world, to a point. My grandmother was always keen to point out people's "faults", calling it honesty, and we all could have done without that, believe me. But four stars in the Airbnb world hurts too much for many hosts to take a chance.
I don't cancel anyone for giving negative reviews, so I wouldn't cancel your guest. But I completely understand those hosts who would. I had a guest a few stays ago who had dished out a few negative reviews. I was very, very careful to make sure she understood what she was getting here, and it turned out fine, but what a worry.
If I saw a glowing review with a reply that said "Thanks for the four stars," I would think the guest was sneaky, and it would be even more of a worry. I like @Lisa723 's and @Mark116 's take on it.
Is "A" going to try calling Customer Service or is she going to give up on Airbnb?