Fewer guests actually staying now.

[Tim107]

I originally booked a room for 3 nights for 4 guests total (which costs an extra $25 a night because of the 4 guests). Unfortunately we are only going to have 3 total people staying there. Can I get a refund or something? 

[Christian65]

You might be able to alter the booking but it depends on the host and the listing @Tim107

Find your booking on the ABB-platform and change the number of people and the host will get prompted right away and will accept your request to change if he deems it fair and fit to his policy.

It all depends on what policy he has and if he wants to accomodate your request. I can't see why not as the amount is so relatively small but ask him and if he's a sensible host he'll accept your request.

[A2]

As Christian said, it's going to depend on the host.  I know that I have a strict cancellation policy because I used to have issues with no-shows and last minute cancelations.  If you're group is renting out multiple beds or rooms, it might cost your host money to alter the reservation.  While these are our homes, there is a business aspect to it.  As for me, if a guest has to cancel last minute, I usually tell them that if the room is booked by another guest, they can get a refund. 

[Alice-and-Jeff0]

The key here is you need to talk to Your host - not all the other hosts on the Airbnb platform (which is what this thread is).  Tell them the situation and ask if they will allow you to do a reservation modification.  Then send the request for a change: https://www.airbnb.com/help/article/913/can-i-change-a-reservation-as-a-guest .  If the host does not agree to the change, you have the option to cancel and be subjected to the cancellation policy of that host.  https://www.airbnb.com/home/cancellation_policies.  

Good luck.