Hi Manfred,
This is a typical issue. This gives you the right to ask the guest for a penalty fee, which you can do through the Resolution Center, even if you are not willing to host the extra people. The guest will mostly reject the the proposal, but if you were very clear with your rules in the description and your advertisement is up to 4 guests, then you will win the case. Either the guest or Airbnb will pay. Don't know if the guest is actually charged thou, I guess its the inssurance that Airbnb provides.
I'am also searching how to contact airbnb by writting a message as I used to do with the old version.
Best regards
Andy