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.
Local ads have been running here in the VA/D.C. area saying Lewis Black will be at the Warner Theater in D.C. on April 21st for a live HBO taping. And Ticketmaster list two show times for Black at the theater on that day, one at 7PM and another at 9: