Even though a hotel may have hundreds of rooms, they will only allocate a percentage of their rooms to any given promotion. If the hotel's allotment of 4th night free rooms are gone, then even though they have space, you won't get the deal. You can sometimes have something open up, but it's not guaranteed. We lucked out, as a couple of days after booking a standard view room at the GCH on the promotion, a view room became available that previously wasn't on offer - we were able to change our package. My suggestion would be to book the hotel if it's where you know you want to stay, but keep checking to see if they either add more rooms to the promotion for your dates, or in case of cancellations. For 4 rooms, though, it will be less likely.