So one of the benefits of Citi Prestige HK is that if you book four nights hotel stay, you will get the fourth night free.

Just going through the process and the offer seems to be legitimate, with a few caveats though.

You call Citi Concierge (who use Ten Group). Citi Concierge in turn get in contact with Mastercard hotels who come back with a proposal. You then book and pay the full value on your Citi Prestige card. A month or so later Citi will then credit your card back with the equivalent of your fourth night stay inclusive of all taxes and fees. However the credit back will be in USD, so you may get messed up on currency rates (because the sell and buy rates by MC are naturally not the same).

Priced out a recent stay and compared it to all the other various credit card offers etc and Citi were willing to match the best rate I found and on top of that offer me the one free night.