4th Night free has typcially been offered after Labor Day until early December. I don't believe they offered it during the busy Thanksgiving weekend however. It can really help justify the cost of staying onsite! A couple of years back I got Paradise Pier for a nightly average of $109 a night which was a phenomenal deal. And $150 a night for the DLH. You can get this through Disney or travel sites like Expedia (I have done it both ways).
As for combining it with an AP discount, I don't believe that would be possible. But I could be wrong! Still, these deep discounts are generally of the "cannot be combined with any other offer" variety.