It's not just for Reactor 4 that TEPCO has been trying to build a support structure in preparation for the removal of fuel assemblies from the Spent Fuel Pool.
Workers (probably from general contractor Kajima and its subsonctractors) have been busy clearing the debris from the operating floor of Reactor 3 and building a platform (or frame, as Kajima calls it), which you see in the July 24 photo that TEPCO included in the monthly report for the Working Group for Decommissioning of Reactors 1 through 4 at Fukushima Nuclear Power Plant.
According to the report, the steel platform was fabricated at the on-site yard on the ocean side (east) and brought to the northwest side of the Reactor 3 building.
From TEPCO's monthly (July 2012) report (page 61):
Radiation levels on Reactor 3's operating floor are supposedly extremely high. Kajima is using remote-controlled (unmanned) cranes and other heavy equipments to remove the debris. The platforms (or frames) will be used to effectively remove the debris by remote control. According to the press release from April this year, Kajima also developed a system (patent pending) to remotely re-fuel the equipments on the platform.