According to US Senator Ron Wyden the unit 4's spent fuel pool may collapse if a major seismic event occurs and the consequences could be worse than the March 11th 2011 earthquake-tsunami -related meltdown accident fallout.
Japan is not moving fast enough in removing the nuclear fuel rods from the units and the U.S. should offer help for speeding things up. Wyden sent letters to Ambassador Ischiro Fujisaki, Energy Secretary Steven Chu, Secretary of State Hillary Clinton and NRC Chairman Gregory Jaczko.
TEPCO and Japan's Foreign Ministry haven't commented the issue.
Link: The Wall Street Journal - Japan: http://blogs.wsj.com/japanrealtime/2012/04/17/fukushima-daiichis-achilles-heel-unit-4s-spent-fuel/