From what Ron D. has said, Season 4 is going to be it, and he's going to finish the story right, and not leave anything hanging. Well, this particular story. He might leave the seeds of something new. But he's said he's going to give this storyline "closure". We may or may not like it , but he's going to finish it.

He didn't want to have it up in the air whether there was going to be a season 5, and have to leave it hanging, then hastily finish if Season 5 was cancelled. And he didn't want to be in the process of finishing it, then have then tell him a fifth season was a go and then have to drag it out.

This "Razor" thing is going to tell a lot of backstory, the rumor/spoiler mill says. It's going to explain how Cain came to be the way she was, and what happened during the original attack. Gina-6 will be in that big time (this comes from people who watch Tricia Helfer's hair -- she was observed
wearing Gina hair a lot during the filming).

And we'll get some story about a young Pop Adama as the young hot shot Viper pilot in the first Cylon war. In that we'll see some of the original style Centurians and raiders that look like the original series. That ought to be fun. Caine was a young girl then and we'll see some stuff that shaped her into what she became.

-Richard