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.
That scene where Sil jumps the guy from behind and the other guy stabbed him reminded me of the scene where Billy Batts was killed for trash-talking in "Goodfellas". Incidentally, both that scene and this show feature Frank Vincent.