Plot
The storyline differs slightly depending on whether the player chooses Mega Man X or Zero. Mega Man X4 takes place in an ambiguous year in the 22nd century (21XX), where humans coexist with intelligent androids called "Reploids". Following the third defeat of Sigma, a second Maverick Hunting group has arisen. The army, called the "Repliforce", is a strict military regime led by the giant Reploid General and his second-in-command, Colonel. Some time later, behind the scenes, General has been meeting with a mysterious figure who plots the Hunters' demise, trying to convince General that the Hunters are dangerous, and will turn on him in an instant. General dismisses the figure, unwilling to betray the Humans. Zero, meanwhile, is plagued by a recurring nightmare: a mysterious figure awakens him, calling him a "masterpiece", then orders him to destroy an unknown target before fading away(not knowing what was shown in 1 fan game and 2 fan games, which resulted in the death of Wily and Bass & Treble from his own hand). As Zero tries to stop the figure, he is paralyzed by a horrible pain, which is followed by visions of carnage. Mayhem breaks out, however, when the Sky Lagoon, a massive floating city, is sent crashing down onto the city below it, killing countless humans and Reploids alike. The game begins here where either X or Zero is dispatched to investigate possible causes of the disaster, only to become entangled in a struggle to save the world.
At the crash site, X and Zero encounter Colonel, and attempt to bring him back to Maverick Hunter HQ for investigation. Colonel denies Repliforce's involvement in the Sky Lagoon destruction, and refuses to disarm out of pride. Zero also rescues Iris, Colonel's kind-hearted sister, who was caught in the mayhem but is otherwise unharmed, and sends her to headquarters. The Repliforce thus begins a movement to claim independence from the Human government and create a nation for Reploids only. Back at headquarters, X is greeted by a rookie Hunter named Double, while Zero rendezvous with Iris. Double and Iris outline the locations of eight Reploid leaders who have sworn their loyalty to Repliforce's coup. Once four of the eight Mavericks are defeated, Colonel issues a challenge to X and Zero, which he either escapes from after defeat (X) or is stopped by the intervention of Iris (Zero). Once all eight Mavericks are beaten, X and Zero are sent to a space port where Colonel guards Repliforce's launch into outer space; Colonel then dies after an ensuing battle. The two protagonists then infiltrate the Repliforce's space station, known as the Final Weapon, which is capable of destroying all human life on Earth. X must fight Double, who is revealed to be a double agent to gain information from the Hunters. Zero is reluctantly forced to battle Iris, who is torn between the ideals of her brother and her feelings for Zero. Once Double, Iris, and General are defeated, Sigma reveals himself as the mastermind of creating the conflict between the Maverick Hunters and Repliforce and announces his intention to wipe out humanity. Additionally in Zero's scenario, Sigma reminds Zero of the time that Sigma led the Maverick Hunters, and of a vicious battle between the two that ended with Sigma becoming the Maverick he is today. In the end, Sigma is destroyed, but reveals he already activated the Final Weapon. General then appears and sacrifices himself to destroy the Final Weapon, allowing X and Zero to escape and return to Earth.
The final scene depends on which protagonist was used to complete the game. In X's ending, he thinks back to the battles he had endured: a misunderstanding Colonel, Double's betrayal, and Sigma himself. Zero contacts X and tells him to return to Earth to rest. With his thoughts weighing heavily on him, X begs Zero to promise to take care of him, should he become a Maverick himself. Zero hesitates, then tells X not to have such thoughts. In Zero's ending, he is wrought with pain and guilt for being unable to save any of the Repliforce members, and becomes heartbroken over the death of Iris. He then wonders to himself if all Reploids are destined to become Mavericks after all.
Gameplay
The gameplay in Mega Man X4 is similar to the previous installments of the Mega Man X series. The player is presented with a series of action-platforming stages that can be cleared in any order desired. In these stages, the player must avoid obstacles like falling debris and spikes, and destroy enemy robots to reach the end of the stages. The player character retains the capabilities from previous entries in the Mega Man X series like dashing and scaling walls. Some levels contain ridable vehicles such as hover bikes and armored-mecha. Each of the eight initial stages contains one Maverick boss, and defeating this boss gives the player a new ability. Every boss is weak to a particular ability, adding an element of strategy to the order in which the player completes the stages.
New to Mega Man X4 is the ability to play through the game either as X or Zero proper (while Zero was available for play in Mega Man X3, he was limited to being essentially a spare life for the player). The two characters cannot be switched during a playthrough. Though both of them go through the same stages, they operate differently and are challenged differently from the terrain. X wields the "X-Buster", a plasma cannon on his arm that he uses to attack foes from a distance. It can be charged to fire stronger shots. A new weapon is given to the player with each boss defeated while playing as X. These weapons have limited ammunition, displayed by a meter next to one's health, both of which can be refilled by picking up power-ups dropped by destroyed enemies. In some stages, the player can find hidden capsules that contain armor upgrades that greatly enhance X's capabilities. Zero is more melee-oriented than X by using a "Z-Saber" sword. The Z-Saber's power and accuracy compensate for its lack of range; this offers the player a different form of challenge against bosses. Rather than acquiring weapons from the bosses (with the exception of his Giga Attack), Zero learns special techniques such as the "Hienkyaku" air-dash and "Kuuenbu" double-jump. However, Zero cannot upgrade any of his body parts in this game.
The player character's maximum health can be extended by obtaining a "Heart Tank" in each of the eight stages. Two "Sub Tanks" can also be found, which can be filled with life energy and then be used to replenish the player's health at any time. Two new Tanks have been added: a "Weapon Tank (W-Tank)", which fills up special weapons; and an "EX Tank", which increases the character's default lives from two to four whenever the player continues from a save point.
