Agree! I have successfully done this last year without having to change room. After suite upgrade is cleared with a WoH agent without mentioning the final night using cert, that is when I will book the cert night online.

At check in with your status, politely ask if Diamond/Glob complimentary upgrade is available on your 4th night. They will most likely tell you that they already saw the back to back bookings and extended the room. If hotel is truly full, you have 3 days to prepare moving room. A little inconvenient yes, free night, not a deal breaker.