Arnie Gundersen of Fairewinds Associates has circulated a YouTube video labelled as showing the Unit 4 explosion at Fukushima Daiichi. The YouTube plays Japanese language audio of two newscasters, while showing television footage labelled as the explosion of Fukushima Daiichi Unit 4. Such footage has rarely been circulated or broadcast. Arnie points out it may be because of the crude quality of the visual footage. Also, the Unit 4 explosion appears less "dramatic" than the Unit 3 explosion. On May 26th, a DOE analyst named Kelly, speaking at a Fukushima subcommittee meeting of NRC's Advisory Committee on Reactor Safeguards, spoke about the theories regarding the cause of the Unit 4 explosion. In the earliest days of the Fukushima catastrophe, NRC chairman Jaczko announced that Unit 4's high-level radioactive waste storage pool had boiled dry for lack of electricity to run cooling water circulation pumps. But Kelly said on May 26th that while a portion of the pool may have boiled dry (apparently there are walled off, isolated sections within the pool), and generated hydrogen gas that then exploded, this theory is contradicted by how quickly the explosion took place, visual images underwater within the pool showing the irradiated nuclear fuel appears intact, and even the radiological tests on the pool water that have been done. A new theory, he explained, is that hydrogen gas from Unit 3 flowed into Unit 4 and exploded. Unit 3 and 4 share a common venting system leading up a single "smoke" stack. Thus, the venting of Unit 3, rather than sending all its hydrogen gas up the smoke stack, may have sent some into Unit 4, which then exploded. If that is true, then the retrofitted "band aid" venting system, designed to prevent a catastrophic breach of the too small, too weak General Electric Boiling Water Reactor of the Mark 1 design's containment structure, not only failed to prevent an explosion at Unit 3, it actually caused the explosion at Unit 4!
Another YouTube video, a Hindi language news broadcast from Tuesday, March 15th, shows what it labels as the Unit 4 explosion, specifically at the 1 minute mark, the 2 minute 30 second mark, and the 3 minute mark, highlighting the Unit 4 explosion with a red circle on the grainy video.