After the One Year War, the girl who would become Four Murasame lost her family, and was adopted by the Murasame Research Lab. It was there were the Federation used and trained her to become an artificial New Type (or "Cyber - New Type"). The scientist in charge of the project erased all her memories from her previous life and just gave her number 'Four' as the name. Namika Coner, the nurse in charge of her, convinced her that working with the Titans and fighting the AEUG was the only way that she had to recover her memories.
After escaping from Kennedy Space Center, Karaba's flagship, the Audhumla reached New Hong Kong for supplies. It was in this place where she meets AEUG pilot Kamille Bidan, and not knowing that he was an enemy, she feels bonded to him, since he is the first person who ever regarded Four as a person in her own right and not as a girl without memories who is bound to become a living weapon.
It is only after the truth regarding Kamille is revealed to Four that she choose to pilot the giant Psyco Gundam and attack both Kamille and the city; her desire of getting her memories back and become a full-fledged person are more important than anything. Kamille desperately tries to calm her down and make her join the AEUG, to no real avail.
After the battle, Four helped Kamille and his Gundam Mark II back out into space. During this events the Psyco Gundam is severely damaged and Four is presumable killed in an explosion.
However, it was later revealed that Four and the Psyco Gundam survived and were taken to the Titans' Kilimanjaro base in Africa. Once there, the Psyco Gundam was repaired and equipped with an experimental new "psyco-control chair", further enhancing Four's abilities to control the Psyco Gundam. Furthermore, Four was forced once again to undergo medical experiments in order to increase her powers, almost losing her memories of Kamille.
At the beginning of November, U.C. 0087, the AEUG and Karaba launched an attack on the Titans base at Mount Kilimanjaro in Africa. Led by Amuro Ray, they were also supported by Kamille Bidan and Quattro Bajeena. It was then that Kamille discovers that Four was still alive. Four finally remembers Kamille when he infiltrates the Titans base to save her. Four was glad to see him, but the backlash coming from her treatment as well as her own mental instability were a huge hindrance.
In the end, Four was forced once again to fight against Kamille in the Psyco Gundam, but eventually sacrificed herself to save Kamille from Jerid Messa.