As far as I know, the 4th reactor didn't catch fire, the building the 4th reactor was in caught fire (briefly). The 4th reactor had been offline since November for inspection and the fuel had been removed and added to the storage pool.
Now what IS worrisome is that the 4th building is being used for rod storage for the other reactors.
Fortunately, the rods are supposed to be stored under water in a storage pool while they cool down (in a radioactive sense) so they can be moved somewhere for reprocessing.
(Still looking up more info, it's early.)
- Grumpy Uncle Gearhead
Now what IS worrisome is that the 4th building is being used for rod storage for the other reactors.
Fortunately, the rods are supposed to be stored under water in a storage pool while they cool down (in a radioactive sense) so they can be moved somewhere for reprocessing.
(Still looking up more info, it's early.)
- Grumpy Uncle Gearhead