There's a possibility that the equipment in the "Pit" on the upper floor of the Reactor 4 reactor building may be emitting the high level of radiation because the water that had covered and shielded the equipment drained to the Reactor Pressure Vessel (RPV) that was in maintenance and to the Spent Fuel Pool when the earthquake hit on March 11.
From Kyodo News Japanese (6/19/2011):
It was revealed on June 19 that it is highly possible that the water level in the "Pit" that stores equipment on the upper floor of the Reactor 4 reactor building decreased, exposing the equipment stored in the pit which is emitting a high level of radiation.
The Reactor 4 was in regular maintenance when the earthquake hit. The cooling system was lost due to the earthquake and tsunami, and the water in the Spent Fuel Pool evaporated and the water level decreased. At the same time, the water level of the Pit, which is connected to the Spent Fuel Pool, probably decreased.
TEPCO said that "the radiation level around the Reactor 4 building is not particularly high, and there is no effect on the environment", but the company started injecting water in the Pit on June 19 to raise the water level in preparation for any future work near the Pit.
The diagram is from Kyodo. The Pit is on the left, the Spent Fuel Pool on the right. The dotted blue line at the top was the water level of the Pit when the earthquake hit.