Fredo Braddock is a character who is seen but never gets any face time or screen time other than at his murder site. Detective Norris is assigned his case in episode 2 of season 4 (@ 41:18 mark), which at first turns out to be an especially unlucky call since there are no witnesses and no murder weapons. Norris and Holly visit Braddock's murder site where an un-named officer tells them that Braddock was still alive when he arrived, but that all Braddock said about his killer was that "it was a guy with a gun," to which detective Holly laughs wholeheartedly at Norris' continued bad luck on the case.
However, later in the episode, Norris comes to Holly and Bunk to excitedly tell them that Braddock was a witness in a Maryland State's Attorney case, and now that he has been murdered the case has been upgraded from a "stone cold who dunnit" (a case that is hard to clear because no one cares about it and there are no leads to follow) to a "red ball," which is a case with a major department priority that gives the officer lots of leeway with resources and overtime pay. Initially Sgt Landsmen tells Norris to investigate the Braddock case as if it were just a normal case without filing the papers to declare Braddock a State's witness. Landsmen says this as a way to avoid creating drama in the department during an election year (the murder of a state's witness was a major plot point early on in Season 1 of The Wire).
Later in episode 2 Carcetti, in a mayoral debate, reveals that not only does he know about Braddock's murder as a state witness, but that weeks earlier, he wrote the mayor a letter asking for greater protection of witnesses (which we saw in an earlier episode in season 4).
Braddock becomes important as he was a witness in the another state's attorney's murder case. His murder and the case to solve his murder become a pivotal plot element in season 4. Braddock's murder case is first assigned per homicide department case flow to Norris, a veteran detective, but is then given to Kima Greggs, who is new to the Homicide division, in the hopes that she will slow down the investigation for political reasons. Burrell was told by Mayor Royce to slow the case down to make Royce look less bad during the ongoing mayoral election throughout the early episodes of season 4.