No, it does not automatically adjust. You will know if you are getting that rate because on the summary page for the quote (where you pick you tickets, room type, etc) it will say January-August Magic Your Way Offer under the resort photo and dollar amount. To get the Buy 4 Get 3 go to the main DisneyWorld.com site and there will be a "page flip" type tab on the top right that shows the Buy 4 Get 3.
Hope this helps!