Have both some good news and some bad news.
The good news is that it is unlikely that the fuel in the pond of unit four was overheated to the point of forming vast amounts of hydrogen. While a hydrogen explosion did occur in unit four. You might ask why this is good news, the reason I think it is good news is that if an event such as a fire which released the short and medium lived radioactivity from the used fuel then it would have also released the long lived radioactivity which includes the plutonium.
While any release of radioactivity is bad, we do have the cold comfort of knowing that the release was a selective distillation of the more volatile elements from the fuel. This means that the very involatile and toxic elements such as plutonium and strontium will have stayed in the fuel. While the krypton, xenon, iodine and cesium are unhealthy these elements are like a fly landing on your nose while the strontium and plutonium are like having your brain sucked out by a giant fly (altogether more disagreeable).
I do however have to question the wisdom of having a single stack which serves units three and four. It has been suggested that hydrogen from unit three may have flowed through the duct into unit four. The hydrogen which built up in unit four then ignited thus causing an explosion. I am hoping that if the explosion occurred then it occurred outside the pool while the fuel was in good condition. These circumstances would greatly lessen the impact of the accident.
The bad news is that unit one suffered a melt down very early in the accident, the shorter the time between when the reactor is shut down and when the fuel melts the more nasty the accident can potentially become. This is because in a reactor shortly after shutdown it will have a lot of short-lived radioisotopes inside the fuel. TEPCO have commented that within sixteen hours of the earthquake the fuel inside unit one melted. TEPCO have commented that the fuel will have reached a temperature of 2800 oC during the melting event. This temperature is a reasonable temperature; uranium dioxide has a melting point in this region.
For more details see earthquake report 83 from JAIF