First..how many people requested to stay? 2 or 4. If it was 2 and you agreed to this, but they brought 4 people then you should raise an claim in the incident centre. You will have to provide suffient proof that 4 stayed - for example in an email you sent to the guests on the platform when you found out.
If they asked for 4, but only booked for 2 people, you should have spotted this on the reservation request. Again raise an incident.
I assume you had a security deposit in place, which will help. If not, then your chances of getting paid the outsanding amount are less.
To be clear: AirBnB did not undercharge your guests - either your guests 'lied' or made a mistake and just entered two guests.
Hope you get it sorted but if your guests are refusing to pay up, then it may be a difficult one to resolve.