|Behind the scenes|
- "I was careless. I did not expect to be pursued. But I'll be careful now. Yes. And you know what? It's better this way. I have the power now! I have the POWER!"
- ―Visser Four to Tobias
Visser Four was a rank held by the Yeerk responsible for the conquest of Leera. After the Animorphs prevented Leera from being taken, he was demoted and given a lowly host in the form of an actor named John Berryman. However, Berryman had located the Time Matrix, and the Yeerk, now in charge of Berryman's body, used the Time Matrix to alter Earth's history to facilitate a Yeerk conquest and become the new Visser One. However, the Animorphs managed to defeat him, while Cassie erased Berryman from existence by preventing his conception. With Berryman no longer in existence, the demoted Yeerk was then given another host body, one that had not found the Matrix, and thus the altered timeline was erased from existence as well.
War for Leera
Visser Four had a close working relationship with Visser Three, and was personally responsible for spearheading the Yeerk Empire's plans to seize the Leeran continent. However, after the Animorphs are snapped to Z-space while in mosquito morph, they find themselves on the Andalite fighter ship Ascalin, heading towards Leera to help the Andalites and Leerans defend their world. The captain of the ship, Samilin-Corrath-Gahar, is revealed to be a traitor allied with Visser Four, and the Animorphs flee the Ascalin. Instructed by Force Commander Galuit-Enilon-Esgarrouth, the majority of the Animorphs decide to enact the Commander's plan to destroy the Leeran land mass, with Ax successfully detonating it before being snapped back to Earth. The destruction of the land mass cripples the Yeerk forces and renders their conquest futile.
- "The Time Matrix."
"What? That's a myth! No such device ever existed."
"Oh, it existed. It exists. It was found by a lowly human-Controller, who uses the name John Berryman. He's an actor. Not a very successful one. A lowly Controller whose Yeerk was, until he lost the battle for Leera, none other than Visser Four. And why did he lose the battle for Leera? Why, because of all of you. Ironic, eh?"
- ―The Drode and Ax
At some point in time, John Berryman discovered the Time Matrix buried at the Abandoned Construction Site. Some time after that, he was infested by the Yeerk formerly known as Visser Four, who had been demoted from Visser rank for failing to conquer Leera for the Yeerk Empire. Once in possession of Berryman's body, the Yeerk discovered that his host body was in possession of the Time Matrix and attempted to use it to rewrite Earth's history in order to facilitate his conquest and become the new Visser One.
Using the Time Matrix
Using the Time Matrix, Visser Four creates a timeline in which modern day Earth still practices slavery, with Cassie being a radical slave-owner who opposes war, Tobias dating Melissa Chapman, who takes Rachel's place in the group as Rachel was sent to a camp to reeducate and remold her into a submissive as she is too bold and aggressive for a female; Jake becomes a Patriot of the Empire with Neo-Nazi tendencies, hellbent on being referred to as "Supreme Leader" while Marco is a friendless cynic whose mother is still alive and lives with him. The Drode interferes, restoring their original timeline personalities and replacing Melissa with Rachel, offering them the chance to pursue Visser Four, retrieve the Time Matrix and undo the damage, albeit at the cost of one of their lives.
Chase Through Time
In order to silence John Berryman, who is quoting William Shakespeare's Henry V, Visser Four travels to 1415 France to kill Henry V during the Battle of Agincourt. However, when attempting to kill Henry V with an arrow, the arrow is intercepted by Tobias, alerting the Visser to the fact that the "Andalite bandits" are chasing him. Visser Four runs away to the church, followed by Tobias and Ax, although he manages to use the Matrix to travel to the Delaware crossing in Trenton, New Jersey on December 25, 1776. There, he warns the British and the Hessians about George Washington's ambush. His actions cause Jake to be killed as well as preventing the United States of America from being formed. Following this, the former Visser time jumps to Cape Trafalgar, Spain in 1805, successfully managing to destroy the ship. Chased by Tobias once more, the Yeerk barely manages to escape, fleeing to Princeton University in 1934, where he plans to kill Albert Einstien, although his prior meddling in the timeline prevents Einstein from being at Princeton; additionally, as he prevented the United States from being formed, the United States is now a British territory with ties to France.
Desperate to finish his agenda and ditch the Animorphs, the Visser travels to various destinations within seconds, hoping to lose his pursuers. Confident that he has lost them, he travels to Normandy in 1944, intent on warning the Nazi Party about the Allied efforts on D-Day. However, his prior actions have prevented the Nazi party or the Allies from existing, and he is arrested by a German-French team, where Adolf Hitler is but a mere driver. He is arrested and taken away by the German-French troops, but the Animorphs launch an attack on him. Rachel drops a grenade inside one of the tanks, killing the troops and paralyzing his host body. The Animorphs then demorph, shocking the former Visser when he discovers that they are humans.
- "John Berryman's parents, John Senior and Theresa Knowlton, will meet right there, today. All we have to do is separate them. They don't meet, they don't get together, they don't have a kid named John, and Visser Four ends up in some other host, in some other place. He never finds the Time Matrix, And none of it happens. Time isn't altered."
With John Berryman's body crushed at the waist down, paralyzing him, the Yeerk escapes his body, although Tobias spots it and Marco throws it into a burning tank. Berryman relishes his freedom, although he realizes that he is going to die. He praises the Animorphs, whom he discovers are just human children for the most part, for fighting against the Yeerk Empire. The Animorphs discuss how to erase the timeline in order to bring Jake back to life, and Ax responds that they would have to prevent Berryman from locating the Time Matrix to undo the altered events. Cassie then asks Berryman when and where his parents met, much to the shock of the Animorphs and Berryman, who realizes that he is about to be wiped from existence. Berryman accepts his fate, informing Cassie that his parents met at a certain location in San Francisco in 1967. The Animorphs travel to San Francisco in 1967, appearing in front of two hippies, one of whom is Theresa Knowlton, Berryman's mother. By distracting her, they prevent her from meeting Berryman's father and thus prevent Berryman's conception. With Berryman effectively wiped out of existence, the timeline is changed, with the Time Matrix still remaining buried and undiscovered while the demoted Visser is given another host body. The timeline's erasure resurrects Jake, and the Animorphs retain their memories of the altered timeline.