4 is in danger because the fuel rods were still being stored in the buildings, near the top of the facility in the holding tanks. Those tanks are essentially swimming pools that circulate cool water to keep the rods cool. Even though the rods are out of the reactor (and are sometimes referred to as "spent" or "depleted", those are really misnomers. These rods are still highly radioactive and still throw off a substantial amount of heat. A single rod (they are about 12 feet long, and about an inch or so thick - internal diameter) is made of a zirconium alloy that you'd think wouldn't have a problem with air, but it does (after all, people wear fake diamonds all the time). This stuff is different though. These rods are hollow. They are sort of like pipes. Inside each rod are these little pellets of highly enriched uranium. When these rods are separate, they are hot to the touch. It's when you bundle them together that they really get hot. Hot enough to turn liquid water into steam really quickly. That's how they produce power.
Well in #4, this pool where they kept all the rods near the top of the facility doesn't have a containment like the reactor itself does. It's a LOT less protected than the core. When the tsunami hit and wiped out all of the power to all of the facilities, the pumps that kept that swimming pool full of rods cool shut down and stopped working. The rods kept doing what they do best, they started making the water hot. It doesn't take all that long (a day or two) to literally boil off all of the water in that pool. That's not a good thing.
The zirconium on the outside of the rods gets insanely hot, and starts oxidizing when it hits air (which is why it has to stay under water). The oxidation process is like iron rusting, but really fast. This oxidation rips the oxygen out of H2O, and suddenly you're left with hydrogen. At that point all it takes is a spark.
4 has lost all of its water in the pool and those rods are fully exposed, generating enough heat to start melting down the outsides of the rods. The heat feeds itself and it keeps getting hotter and hotter. Eventually, the uranium will get hot enough to melt as well (if they don't get those things cooled off soon) and you can imagine what molten metal does. It will pool at the bottom just like any other liquid would. This stuff is highly radioactive, and unlike a reactor core, this stuff has relatively easy access to the open air. That's why #4 is such a bad thing. Once uranium gets hot enough to melt, it's obviously already burned off the zirconium sheath and, they call this radioactive lava at the bottom, "Corium". It can eat through concrete and cause all sorts of problems.
