This is where TEPCO and subcontractors have been working on to build the foundation that would support heavy columns like the first two that were just installed on January 8, 2013.
The foundation building work has been described by some as the evidence that the Reactor 4 building is collapsing.
From TEPCO's Photos and Videos Library, 1/8/2013:
The steel frame over the Reactor 4 building that will suspend the crane to remove fuel assemblies will be completed by October this year, and TEPCO plans to start removing fuel assemblies from the Reactor 4 Spent Fuel Pool in November 2013, according to Mainichi Shinbun (1/9/2012). That will be one year earlier than previously planned.