Short article:
http://www.nasaspaceflight.com/2011/09/japanese-h-2a-launches-new-igs-military-satellite/
A third optical satellite – IGS 4A – and a second radar satellite – IGS 4B – were then launched on 24 February 2007. IGS 4A is classed as a advanced and experimental optical satellite, based on the Synthetic Aperture Radar (SAR) technology.
AFAIU, a SAR antenna is used on Radar satellites not on optical satellites, which typically use telescopes.
Also, according to the
Japanese Wikipedia page on IGS (different from the
English version 
!):
IGS-4A was "Radar-2" not an optical satellite while IGS-4B was Optical-3-Demo, not a radar satellite.
IGS-5A launched on Nov 28, 2009 was Optical-3.
AFAIU, the "Optical-4" launched this morning would correspond to "IGS-6A"