@Robson46 I understand that this seems very unfair to you, and I agree that they should notify you before charging you. But it is up to the host to keep their calendar dates, their pricing and their booking settings up-to-date. The reservations that came in were instantaneous because Instant Book is the default setting when you set up a new account, but you could have changed that to Request to Book, which is always a good idea for new hosts until they learn the ropes (it's a big learning curve for all of us).
All the information re these policies is available to be read in the Terms of Service and in the Help pages for hosts on the Airbnb site. When I first signed up to host, I made sure to read through all of that before submitting my listing. Unfortunately, many new hosts seem to put the cart before the horse and go live with a listing without bothering to inform themselves of how it all works.
Sorry this happened to you, $400 is a lot of money to lose, but you could try contacting customer service and pleading ignorance as a new host and see if they'll give you a pass on this.
(Keep in mind when you talk to customer service that your goal is to get them to help you, so telling them they are abusive won't get you anywhere- try to stay polite and on-point, the CS reps don't make policy, they are just lowly employees who get yelled at all day by frustrated hosts and guests)