Sorry not terribly clear from your comments. Did they leave the gas on, or were they smoking pot in your apartment. I don't understand why if they were two hours late checking out, why you agree to leave and return. You should have told them to leave immediately.
You need to contact BNB now (all contact 1st post in the forum you are posting in). Let them know that four guests stayed rather than three, they were smoking pot in your apartment and there is lots of damage. You can also ask them for extra fee for them leaving after your agreed check out time.
You also need to flag this by going into the booking confirmation and contacting BNB through their resolution centre.
You don't have anything in your house rules about no smoking or no extra guests allowed to visit or stay, so add these in now.
Presuming you are allowed to sub let your apartment, you may be better with a long term tenant rather than using BNB