Is it really that big of a deal that you didn't get 4 beds? 3 beds and 1 couch does accommodate 4 people, albeit not as nicely as 4 beds. I would have been a little miffed and left a review stating my experience.
I would expect AirBNB to take responsibility given that the incorrect info is on their website and you pay pretty hefty fees to use the service. (AirBNB charges about 15% to the tenant and 3% to the host so a total of 18% - not bad ;))
Not to mention there are plenty of nightmare stories surrounding hotels, they are not perfect either.