We were able to do this...we booked the "room only" rate for 2 nights in January through the Special Offers section on the Disney website...that discount was good for about $100 a night off our room at the Contemporary. AND then we added 2 day base tickets AND we added DDP - all that was available on-line. First it quoted the room only rate, then allowed you to select ticket choices & dining choices. And through all that, it kept the discount on the room. Which was about a 27% discount. We later upgraded to DxDDP (over the phone.) AAA would have given us a 20% discount on the room.
To the original question, 4 for 3 when you can only stay 5. That's exactly what I looked into when the offer first came out because we were thinking of doing 5 days at WDW in January. The answer was if you buy a ticket to make it a package, you can buy just a 5 day ticket, but if you then add the DDP, you will pay for all 7 nights on the dining plan because it's sold for length of stay which would technically be 7 nights on this deal. So even if you check out after 5 nights, you still paid for 7 nights of dining (no refunds for unused meal credits - and you have to pay for the DDP in advance.) That's what I was told anyway. So this idea would only work out if you think you could use 7 nights worth of credits in 5 (if you're thinking about doing some 2-credit table service meals like signatur dining or dinner shows, it just might work out for you.) As others have said, the only way to know for sure is to do the math. If they will let you book it as pay for 4, get 1 free then you should be able to pay for just 5 nights of dining, but when I called about it, the CM said the system wouldn't let her book it as 4 + 1 free...