The SDF-4 Liberator was a Human Battlefortress commissioned by the Robotech Expeditionary Force and was the Flagship of the Earth Reclamation Force. It had the callsign Command One during the Earth reclamation assault.
The first known appearance of the ship was in 2023, when part of the Robotech Expeditionary Force joined The Sentinels on their mission to free worlds occupied by the Invid. The Farrago, an Ikazuchi-class ship, and a Garfish-class ship, and other ships were also present. (Robotech II: The Sentinels: Farewell)
This vessel was part of the fleet that was dispatched to liberate the Earth during the Third Robotech War after the planet fell under Invid control. It became the command ship that was part of the final offensive on the planet in order to free the Human homeworld from the alien domination with the command being under Gunther Reinhardt. Following heavy casualties, General Reinhardt ordered the deployment of the Neutron-S missiles from the SDF-4. The warheads were, however, destroyed by the Regess who departed known space with the entire Invid race along with all stores of Protoculture on Earth. As such, the SDF-4 along with the surviving fleet docked at Moon Base ALUCE in order to determine their next course of action. (New Generation: Symphony of Light, Robotech: The Shadow Chronicles)
Behind the scenes
In the novels' continuity, the ship isn't called an "SDF" but is instead the flagship of the Third Earth Reclamation Force. In The End of the Circle, a battlefortress dubbed the SDF-4 was being built. But due to Harmony Gold's 2005 retcon, the book series are no longer canon.