I am not 100% certain, but I believe the 5th sleeper option is for those parties of 4 that want the Murphy bed. Families that have 2 kids of opposite sex sometimes want 2 different sleeping surfaces so the kids don't have to share a bed. Now with this new category, the can actually book a standard room with a Murphy bed instead of it just being a request.
So if you're a party of 5, and book standard, you will get a room in Alligator Bayou section that has the Murphy bed without paying for the upcharge of the new category.