The Fourth Robotech War is a conflict that began in 2044 between the Robotech Expeditionary Force and the Haydonites in the Battle of the Omicron sector, which was concurrent to the last major battle of the Third Robotech War.
The Haydonites, who had been the REF's allies during the Third Robotech War, sought to purge Protoculture and all those who had come into contact with it. The technology given to the REF by their so-called allies turned out to be a trojan horse and the REF was swiftly crippled. However, quick tactical thinking by Captain Vince Grant at the Battle of Space Station Liberty annihilated the Haydonite armada, along with Liberty. Despite the destruction of the enemy fleet, the REF expected to see the Haydonites again, and began swift preparations to locate and rescue the missing SDF-3 which held the Protoculture Matrix and their only chance at holding off another attack. (Robotech: The Shadow Chronicles)