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"