In early January 2007, Iridium 97 (27450,2002-031A), up till then a spare satellite in orbital plane 4, entered the operational constellation, evidently to replace Iridium 36 (24967, 1997-056C). However, Iridium 36 remained close to its nominal position in the constellation, and it was initially unclear whether Iridium 36 had failed completely. However, a visual report in early March 2007 seems to confirms that Iridium 36 has failed.
This left orbital plane 4 with Iridium 51 (25262, 1998-018A), probably a partial failure, as its only spare satellite. Iridium 98 (27451,2002-031B) was originally also spare in orbital plane 4, but was in the process of migration, presumably to orbital plane 6.