The second week of September is generally a good week for availability. (I'm assuming you mean the week of the 11th.) Sunday through Tuesday might be a wildcard because of the Expo (a lot of folks will hit the parks the days before, during, and after the Expo) - but everything after that should be okay.
In general, we tend to suggest just buying a 5-day parkhopper and upgrading to an Enchant key at the very end of your trip. That would guarantee you entry for all five days. You'll miss some of the discounts, but you won't have to stress about losing a day. (Reminder: under the current system, upgrading to a Key will cost you your existing reservations, so it's usually best to do that after you leave the last park. Hopefully, they'll fix that in the next six months.)
If you're willing to risk it and get an Enchant key before your trip, I think the smart play would be to book any Friday, Saturday, and Sunday on your itinerary before anything else. Those days will fill up before the others do. So, for example, if you're going Wednesday through Sunday, leave Thursday open and book it as soon as you get into the park on Wednesday.
But, yes, they do tend to add availability the night before. In theory, it shouldn't be all that hard to get the extra day.
The honest answer - you probably won't have a problem getting the fifth reservation. But, unfortunately, the best we can give you is probably. There's no real guarantee.