I am thinking we'll get the fifth revealed in ep. 9.
I thought I read somewhere it won't be until the final ep of the series. I'll have to see if I can find the source of that. I would think that the 4 would be revealed to the humans/Cylons before we figure out the final one.
Pretty sure I read the opposite somewhere, but I think it was in EW and we toss those weekly. I recall the writers saying they wouldn't make us wait till the end - that it would be revealed sometime mid season.