I'm in the same boat as you. I booked two hotels in Japan for middle of April using the 4th night free benefit last year. Because of the COVID-19 pandemic I am going to have to cancel and have the same question if I can rebook using those lost 4th night free for a later date this year. Both of my hotels are fully refundable up until 3 days before arrival so I know I will get my money back. What I want is to be able to use those "lost" 4th night free when I booked in 2019 and apply them for hotel stays later this year without it using up my current 2020 4th night free benefit. Its one of the reasons I applied for the card last year and paid my hefty annual fee. Let me know if you hear anything. I will post on my experience too as I approach my travel dates. I haven't canceled yet just in case something miraculous occurs and this all clears up (very unlikely).