It came down to the 11th hour, but both Variety and Hollywood Reporter's websites are saying that BL has been picked up for a 5th season. I think the negotiations had to do with ABC buying out "Life on Mars" from David E. Kelly, and in exchange, Kelley wanted at least another season of BL.
From what I've read elsewhere, it's a full-season, but ABC has the option to trim it down to 13 episodes.
Same general format as the Harry Potter Deadpool. 1) Call whoever you think is going to die by the end of the series. In addition, if you want you can reverse it and also call someone who is already dead ending up alive at the end of the series.