I've read a couple of fan theories on who the fifth is, and there's one in particular that seems more plausible to me. And that person is...
Ellen Tigh
Now hold on, let me explain:
1. Most obvious reasoning is that she appears as a vision to Saul Tigh like Caprica Six does with Baltar.
2. D'anna Biers said in Season 4 that the final member was not on the fleet. Naturally, Ellen was dead at this point.
3. During her vision, D'anna apologised to the 5th member. Perhaps this was an apology based on Ellen acting as a spy for the Cylons which lead to her demise on New Caprica
4. Cast your mind back to the episode 'Tigh Me Up, Tigh Me Down' in Season 1 where Ellen made her first appearance. And initially it was quite odd right? She'd been separated from Col. Tigh and assumed killed in the initial attack? I seem to remember there was some suspicion, but was just accepted because Col. Tigh was pleased to see her. And come to think of it Baltar tested her blood and told them all that she was human, however when 6 asked him for the truth he said he wouldnt say and he was gonna keep it a secret and thats the last we heard of it.
5. I've heard that nobody listed in the 'Last Supper' pic (
http://scenescreen.files.wordpress.com/2008/05/battlestar-galactica_lastsupper.jpg) is the last member. So you begin to think of who else it may be and you think Tom Zarek, Dee, Gaeta but remember, D'anna said the final member wasn't in the fleet. So really the only other person could be Zak Adama, but due to his early demise D'anna would have no encounter with him.
Of course the big contradiction in my reasoning is that Ellen is dead. But I'm going out on a limb to suggest that the final five download into another body again when they die, but we have no idea if that is true.
Knowing BSG though, they'll find something to completely twist my conceptions around and leave me stunned, guess that's why I love it so.