To support this hypothesis, TEPCO released this diagram on August 27 press conference, which shows the surface radiation levels of the gas filters. Highest radiation is at the filter closest to the exhaust stack, lowest radiation at the filter closest to the reactor building.
The radiation seems low though, considering the amount of hydrogen it must have taken to render the building in a wreck, and therefore the amount of radioactive materials that came with the hydrogen gas must have been high. TEPCO's Matsumoto's explanation was because the gas from Reactor 3 came through the Suppression Chamber - i.e. wet vent. (Matsumoto's answer is via Ryuichi Kino's tweet; Kino is an independent journalist covering TEPCO from the day one of the crisis.)
Reactors 1 and 3 exploded with explosive sounds that were clearly heard and felt in the nearby towns. But there wasn't any such sound when Reactor 4 supposedly exploded. I say supposedly, because no one seem to know for sure exactly what happened, including TEPCO.
Cabinet Secretary Edano at that time said "explosion" of some sort at Reactor 4 on March 15, but the media report was focused on the mysterious "fire" that was spotted by a TEPCO employee who didn't bother to report to the local fire department (as he should have). The fire died down on its own, but then another fire flared up again the next day.
The initial report on the Reactor 4 incident was that one or two wall panels in the northwest corner of the reactor building were blown out, and there was some damage on the roof. It sounded minor.
Then all of a sudden we were shown the wreckage. One or two wall panels blown out? It was more like "one or two wall panels remaining".
Read my earliest post on Reactor 4, on March 15. No photo was released that day.
Here's what TEPCO has put down in the plant status report (page 16). TEPCO does say "explosion" but that's decidedly not what was reported back then:
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. At 9:38am, the fire near the northwest part of 4th floor of Unit 4 reactor building was confirmed. At approx. 11:00am, TEPCO employees confirmed that the fire was extinguished.
At approx. 5:45am on March 16, a TEPCO employee discovered a fire at the northwest corner of the reactor building. TEPCO immediately reported this incident to the fire department and the local government andn proceeded with the extinction of fire. At approx. 6:15am, TEPCO employee confirmed at the site that there were no sign of fire.
It's about time we get the much better, and precise picture of what happened in the early days of the crisis, now that we've been better educated since March 11.