It has only be known now (source: Fukushima Diary), more than a year later, that, besides the explosions of reactor 1 and reactor 3, reactor 4 also exploded and had two fires. That explains why the building is so extremely mauled, something that had not been explained yet. Reactor 4 is often considered to be the most dangerous (if any has to be) because while the others "only" have the fuel, this one was used as storage for spent fuel, which, in spite of the name, is the most radioactive of all - and also the amounts are much much larger because it includes the fuel used in all the plant and maybe others for decades.
Fukushima Daiichi is not the only nuclear plant causing trouble, Fukushima Daini is as well (and little is known of its state) and also others. Even the newly reopened Ooi NPP had an alert because of too low water level, whose cause is at the moment unknown.
Against the law and their promises, TEPCO did not disclose the alert until long overdue, highlighting the dangerous secrecy of the mafioso electric company of Japan.