Corporal Timothy Yorba was a Sentinel who participated in Operation Iron Fist. His team was led by Lt. Nathan Hale, and their mission was to infiltrate a Chimeran facility and acquire a nuclear fuel cell. The team infiltrated the base through three captured Stalkers and later using the machines in destroying the Chimera's defenses once their cover was blown. Yorba had placed demolition charges on the Stalkers as the Sentinels prepared to enter the base, just in case they desperately needed to destroy the vehicles in order to create a diversion. He also utilized his expertise to open the doors leading into the base. Yorba was wounded during a firefight on the roof of the facility, but was able to continue fighting due to his regenerative abilities, and survived to complete the mission.
Following the collapse of the United States and innoculation of Hale Vaccine, Yorba participated in an assault on the Chimeran hub tower in New York City. Prior to the assault, Yorba was bitter to the news of Nathan Hale's death at the hands of Joseph Capelli, whom Yorba shown some hatred for the former Sentinel. Yorba was later subsequently killed in the assault.
Yorba is described as having black hair, brown skin and a round face by Nathan Hale. Hale also describes Yorba as being very cheerful, which is evident when he blows open the doors to the Chimeran base and says "Open sesame!"
Yorba was referred to as Echo-Four several times during Operation Iron Fist, which means that he was likely an early member of Echo Team.
Tim Yorba was among of Sentinel who survivied after Hale's death and innocultaed with the Hale Vaccine ad no lorger need the Inhibitor.