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.
I've been an absolute sucker for the Looney Tunes my entire life, so the idea of the Looney Tunes in a sitcom setting was worth a shot. And it's...well...it's not terrible, which is the nicest thing I can say about it.