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 thought it was just his ability to nullify anything that's happening to him, like he did twice previously with the drugs. In this case, it didn't just allow Sylar to step out of time with Hiro, it turned off Hiro's powers completely.