Update: 20:50 I can personally confirm that this IS a Locke-Centric Episode.
Unlike Ausiello, who never credits others site, we'll actually credit them them for this getting the title of Episode 6.04. It is called "The Subsitute" and is rumoured to be Locke-Centric.
Source: EW See Ausiello it's very easy to create links to sources :)