Contrary to what you may have heard, the rumor of the collapse of Reactor 4 is, for now, just a rumor. What you have instead is the result of a planned cleanup/demolition by TEPCO.
Several readers of my blog sent me the link that shows photographs of Reactor 4 at different times in December and links to a post that claims Reactor 4 is falling apart.
Alarmed, I started looking for information. There was absolutely no news or rumor of that in Japan, not even in the Internet message boards on the Fuku-I disaster. I looked at the photos again. I noticed that part of the ceiling trusses were gone, and it looked they were removed systematically.
Then I remembered the tweets a few months ago by the worker who tweets from Fuku-I, saying the debris clearing in Reactor 3 and Reactor 4 upper floors were ongoing. On checking his recent tweets, sure enough, it was a planned take-down of part of walls and ceiling in Reactor 4:
From December 13 tweet:
In Reactor 3, they are removing the debris on the operation floor using a crane. It will take a long time still. In Reactor 4, they removed the failing beams, columns and ceiling trusses from the south side of the operation floor. It looks cleaner now. They also removed the wall panels on the east and west side of the southern end of the operation floor, so there is no more wall panel that had the letter "4" for Reactor 4 on the top floor on the east side.
Then one of my readers just posted TEPCO's Progress Status report on November 17, 2011 in the comment section. On pages 26 and 27, TEPCO shows the cleanup/demolition work in Reactor 4 in details:
So the long story short, no it is not falling apart.
(H/T anon reader)