Well, funny you should mention that

It was 5,000 initially when I checked in, which I did think was slightly high for a cat 4. When I got back home, I noticed on my statement that it said "CAT 4 SPECIALTY UPGRD ONLY 5N", so I called SPG to query this and asked if the 5N meant 5 nights. The agent told me, sorry someone made a mistake and refunded the charge and put a new one for "CAT 4 SPECIALTY UPGRD ONLY 1N".