Actually, I had another thought - if the last Cylon was in the fleet, wouldn't they have heard the music that the other 4 did, too? Surely they would have - unless the music was only on Galactica (but why? Apparently it's transmitted for some reason to awaken these last 4 or 5) - or the last Cylon is different AGAIN than the other 4 . . . already the plot is getting a bit complicated, with Roslin, Starbuck, and Baltar seemingly having 'special' roles but not seeming to be the 'last' Cylon . . and then we have these 4 special Cylons . . . another 'special' Cylon would seem to complicate things even more.

But who knows, maybe that's the plan anyway. Complicate things beyond what we'd expect . .