I missed this news, from over two weeks ago. A TEPCO press release from April 30th reports efforts to reinforce "severely damaged" walls supporting the unit #4 spent fuel storage pool:
Since the walls of the reactor building, Unit 4 are severely damaged, we have been evaluating the seismic capacity to confirm the soundness of the building. As the result of the evaluation, we have decided to install support structure under the spent fuel pool to enhance safety of the building.
There is a brief mention of the hazard in a Reuters article:
Officials also remain worried about structural damage to the No. 4 reactor and whether its storage pool for spent fuel rods has sufficient support. A strong aftershock could topple the structure and spill and scatter radioactive fuel on the ground, compounding the crisis, experts have said.
[NISA spokesman Hidehiko] Nishiyama said that Japanese officials "don't believe it is in danger of immediate collapse," but want to shore up the No. 4 reactor with new steel girders and cement.