"At approx. 6:00am, March 15, an explosive sound was heard and the damage in the 5th floor roof of Unit 4 reactor building was confirmed."
I always assumed that the explosive sound was from Reactor 4. Then a few minutes ago I was reading the same report in Japanese, and what TEPCO says there is slightly different:
At approximately 6:00am, March 15, an explosive sound happened [somewhere] in the plant compound. Later, a damage in the 5th floor roof of Unit 4 reactor building was confirmed.
It does not say the explosive sound came from or near the Reactor 4 building. (Besides if I translate literally, it doesn't say "explosive" at all; it says "big sound" or "loud noise".) It lets readers naturally assume that the big sound was from Reactor 4 because of the damage you can see today, but technically TEPCO isn't connecting the two events - big sound and the damage in the Reactor 4 roof. ("Damage" is such an understatement.)
Checking the explosive events for Reactors 1, 2 and 3, TEPCO's Japanese and English reports both say "near Unit 1", "near the suppression chamber [of Unit 2]" and "near Unit 3".
Companies like TEPCO, as well as government ministries and agencies, craft their language very, very carefully. Particularly for the event like this nuclear accident with huge liabilities TEPCO has been extremely careful in not saying what it doesn't want to be construed in a wrong way.
It is possible that I am reading too much into this, but I do believe that TEPCO does not quite know what exactly happened in the early morning of March 15. It is clear that some explosive event happened, but TEPCO doesn't quite know where it happened. So in the Japanese report it said the explosive sound was heard somewhere on the plant, and in English report they decided to drop the location altogether.
Whatever happened that morning, it released a huge amount of radioactive materials that spread all the way down south to Tokyo and beyond.