Update 6:51 AM: A depressing read here from the New York Times sizing up the situation.
Update 5:51: And now according to Kyodo Wire, there are concerns at #5 and #6.
Update 5:03 AM: According to broadcaster NHK, Japan's Defense Forces have aborted its latest plans to spray water on reactor #3.
Evidently it was deemed too dangerous, due to radiation. Plans are being drawn up for a ground attack on the nuke plant.
Update 4:49 AM ET: Nouriel Roubini's shop RGE Monitor (via @tracyalloway) has sent out a note on the crisis, predicting it gets upgraded from a 4 to a 5. The note is authored by Mikka Peneda:
This year marks Chernobyl's 25th anniversary, and how ironic it is that the world has a new nuclear emergency on its hands: Japan's Fukushima power plant, operated by TEPCO. The situation at Fukushima continues to worsen, with explosions at two more reactors and the radiation released surpassing that of Three Mile Island. The 40-year-old reactors, designed by General Electric, were due for decommissioning at the end of this month.
The Fukushima nuclear incident will likely be upgraded from a level 4 to a 5 on the International Nuclear and Radiological Event Scale. The scale runs from 0 to 7—the most severe. The incident will remain "an event with local consequences," although this excludes the consequences for the expansion of nuclear power generation around the world. Three Mile Island was a level 5; Chernobyl was a level 7—the only level 7 event so far.