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:
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.
Received notification of changes in the mail but it was slightly misleading. It said that the 4th night free benefit remains but in the fine print it says refer to the T&C for changes.
The T&C also say that you can't break a stay with just one night in order to get two lots of the benefit anymore.