Dana knows the 5th is not in the fleet
As I said, it could have been Helo or anyone on the baseship.
The alternative is that it is Starbuck. It was hinted that though Dana was boxed, they brought her back on occasion to try and get her cooperation on matters. Perhaps she was aware of Starbucks crashing into that swirly thing and presumed death, but not aware of her return, and so figured she was dead. Hence 4 in the fleet.
If there are indeed "good" cylons to come in the series, it would kind of suggest starbuck, she died, was downloaded on/near earth, and put in a replacement viper before being sent back home? Starbuck...herald of destruction...could that just refer to the destruction theyre now seeing on Earth?
And come on, lets discuss this instead of just spouting rumours found on myriad TV and fan websites...