The title says it all: the final Cylon, aka the fifth, will be revealed in the second part of Battlestar Galactica Season 4. Take a look to this trailer that was shown after Revelations, the last episode of part 1:

Battlestar Galactica Season 4 Part 2

Who do you think is the final cylon? Part 2 of Battlestar Galactica Season 4 will be epic!
:)