The fourth game in the X saga was developed for the Sega Saturn, Playstation, and PC and released in 1997 (PC 1998). Like X3, the game featured Zero as a playable character but was the first game to do so from the very start. The game allows players to play the entire game as X or Zero, both obtain special abilities after defeating the main eight bosses, but only X could find hidden upgrades. The upgrade system remain essentially the same, but unlike X3, the upgrade control chips for X were removed and instead allowed X to have two types of X-Busters: either four charged shots or one slightly more powerful shot which leaves a small energy ball when it hits an enemy, remaining for a few moments to deal additional damage. When defeating the bosses, X continued to gain their special attack as an ability, while Zero learned techniques, which either enhanced his jumping, sword, and gave him a special attack like X's Giga attack.
Additionally, this game featured the ability to use cheat codes to gain special armors for both characters upon start. For X, he can gain his Ultimate Armor by using a special, platform specific game code at the start. When he finds his first capsule, he gains the entire armor instead of the individual upgrade. Zero's armor is black and gives him silver hair, which is given immediately at the opening stage after the cheat code is entered. Unfortunately, his armor is purely for aesthetics and provides no actual benefits.
Lastly, this was the first Mega Man game to feature movie cinematic cut scenes to tell parts of the plot. The game did use a large portion of text dialog during the game, but also contained short, anime style cut scenes specific to both X and Zero's storyline.
Since the events of Mega Man X3, the Maverick virus has still not been fully contained and the Maverick Hunters are still busy as ever. To help them in their efforts, a special army of reploids known as Repliforce was created specifically to address the contagion and assist the Hunters with infected reploids. Led by two advanced military reploids, The General and The Colonel, this force marked the most advanced army the world has ever seen. During this time, Zero helped the Repliforce while it was being established and became involved with the Colonel's sister, Iris.
A period of time elapses and no real improvement in controlling the Maverick virus and reploids is seen. Eventually, the Repliforce is seen as a failure and is set to be dissolved. Rather than face disbandment, the General and the Colonel decide to declare independence and to take the Repliforce into space to live as independent nation of reploids in peace with the Earth. Unfortunately, events begin to unfold during this declaration which involve a series of misunderstanding between the X-Hunters and Repliforce and a mysterious reploid who is instigating these attacks.