In episode S4E04 of Breaking Bad, Hank is helping another detective when he says something about a "dead Heisenberg" which messed his investigation.
So, who is this dead Heisenberg?
There is a scene in S4E05 "Shotgun" where Hank is talking to Tim Roberts a detective at the Albuquerque Police Department:
Hank: You know, uh, Tim, look, I appreciate what you're doing, really-- Uh, you know, keeping me involved and all, but, uh, you know, finding this guy Heisenberg dead like this I have to say, it, uh kind of feels like closure to me, you know?
They are talking about the investigation into the murder of Gale Boetticher, who was secretly working for Gus Fring, as assistant to Walter White. The DEA are assuming that Gale is the mysterious Heisenberg .