I think the 3rd/4th/5th nt free promos are based on how the hotels quote the daily prices and CWT works off of that. Some hotel chains say 4th nt free, but it essentially is just a 25% off every day. Then some hotels would have a daily rate for each day and then 4th nt is just $0. If this is the case, I feel like CWT will give you $0 aka whichever is the 4th nights rate.
The reservation I just booked was a 5th nt free promo (I booked 6 nts). Each night was $399/nt but hyatt site was showing $333/nt basically calculated by 399x5/6. At first I was expecting a $333 refund since I thought it was just $333 across all nights (since that was how Hyatt was showing the per night price) until I received the CWT/Citi Prestige confirmation email which showed the daily break down on the rate which was $399 for days 1-4, $0 day 5, $399 day 6. As a result, CWT willl be crediting me $399 (instead of the $333 that I was expecting). So in my case, this scenario is clearly in my favor (saved another $66!) which was a pleasant surprise. ^