I am curious if I can adjust a stay after booking without canceling and rebooking.

The dates of my stay are going to be a 4 day period over 6 possible nights, I will not know the exact nights for 2 months. I don't want to risk the F&F deal selling out at the hotel I am looking at.

Can I book the room for 6 nights, and later contact them to decrease my stay to the 4 nights I actually need?

PS if i recall, I do not get charged upon booking, is this correct?