I noticed this on the Citibank Prestige T&C. I haven't received anything in the mail / email about this as far as I remember:
b. For reservations made on or after March 15, 2017, Principal Cardholder will receive a statement credit based on the average nightly rate (excludes taxes) of the first four-night stay in the same hotel property, upon booking by the Citi Prestige Concierge for consecutive four nights. Hotel imposed fees and incidental guest charges are the Principal Cardholder’s responsibility.
So therefore, as per the US Citi Prestige card, the 4th night free will now be a 25% rebate excluding taxes which sometimes might be in your favour, and sometimes not. I'm not sure if the reference to "reservations made on or after 15 March" means the stay is after 15 March, but my interpretation is that you can make reservations before 15 March for subsequent stays, and the old rules will apply.