The Fourth Empire is the main villainous force from Sega's Super Scaler game Galaxy Force / Galaxy Force II.
The Fourth Empire is a group of galactic conquerors, led by the tyrannical Halcyon, which seeks to conquer the entire Milky Way. In order to counter the Fourth Empire, the Space Federation was formed, leading into a intergalactic war which shows no signs of stopping. The latest target of the Fourth Empire's galactic domination is the peaceful Junos solar system, comprised of five planets and located at the edge of the galaxy.
Halcyon orders a full scale attack into Junos, which is easily conquered and left into ashes from the attack, after which they immediately begin the construction of gigantic fortresses on each planet, with would give them the strength to finally conquer the entire galaxy. In order to stop the Fourth Empire and reclaim Junos, the Space Federation creates the Galaxy Force, a squad of around 2000 of the galaxy's best pilots, each one piloting a TRY-Z Quasar, the most advanced starfighter created by the Space Federation. With the Galaxy Force formed, they are immediately dispatched into Junos to fight against the Fourth Empire's forces and reclaim each of the five planets, before launching an attack on the Green Death, the Fourth Empire's main base.