Hello Jennifer
I think the answer your looking for is dependant on your cancellation policy once the guest has booked. Mine is strict which means the guest pays more money if they cancel shortly before their dates. You could choose flexible or moderate and the airbnb page on this explains the differences well. As for your changed request with this guest it depends on you. If you don't want to accept the booking you do not have to for a new request. As long as you don't decline too many bookings there should be no problem or negative effects on your listing place.
The one thing you must not do is cancel the booking once you have accepted. So if you have accepted the 4 days in this case don't chew on it for a while then cancel. That will affect your listing in a very severe way.
I believe the only reason you should cancel is if you feel you are in danger or the roof collapses, or there is a flood, war, plague or other catastrophic act of God. Again I would hunt through the airbnb guidelines thoroughly to get the exact info you need. I am not an expert. Hope that helps. Good luck....D