Mega Man X4 allows the player to choose between the two mechanoid "Reploids" protagonists at the beginning of the game: X, who uses an arm cannon; or Zero, who wields an energy blade.

Taking place in the 22nd century, the Mega Man X series is set in a society populated by humans and intelligent robots called "Reploids," which are branded "Mavericks" if they commit crimes.

Two Maverick Hunters, X and Zero, fight against a Reploid army called the "Repliforce" who are waging war against humanity to earn their independence.

Development of the game started due to Capcom's desire to explore the franchise in a new hardware which proved to be difficult as they aimed to make the two protagonists unique when playing.

Critics praised the ability to play as either X or Zero, a concept many found to expand upon the then perceived exhausted gameplay formula of the Mega Man X sub-franchise during the 1990s.

Rather than acquiring weapons from the bosses (with the exception of his Giga Attack), Zero learns special techniques that do not require ammo such as the "Hienkyaku" air-dash and "Kuuenbu" double-jump.

[3] Mega Man X4 takes place in an ambiguous year in the 22nd century (21XX), where humans coexist with intelligent androids called "Reploids".

One of them, ex-Hunter Magma Dragoon, reveals that he caused the Sky Lagoon incident for a chance at fighting X and Zero, but hints that he was working for someone other than Repliforce.

Originally, the Mega Man X3 team had no plans to provide a sequel until their superiors from Capcom aimed to try the X series on new hardware.

In order to get the approval to make Zero playable, the developers gave him special moves based on the Street Fighter fighting game series to compensate for lacking X's powers.

[13] Instead of presenting Repliforce as blatantly evil villains like Sigma, the writing staff decided to leave them some "moral leeway".

[13] Since the main theme involved Reploids fighting each other, Capcom decided to avoid featuring human characters, calling it "Robots and The Future".

[13] Artist Haruki Suetsugu did not design its characters as he would do for later games in the series, but was given drafts in order to draw illustrations for promotional purposes.

[13] An unknown author was responsible for designing X's secret "Ultimate Armor" featured in both the game after inputting a cheat code and as a Japanese Bandai action figure.

[17] Mega Man X4 was initially developed as a Sega Saturn exclusive and slated for a June 1997 release,[19] but it was delayed and made multi-platform.

[23] The American localization of the Mega Man X4 PlayStation version was originally put on hold after Sony Computer Entertainment America denied Capcom permission to release it in the United States,[24] reportedly due to their policy against 2D games.

[26] According to a Capcom spokesperson, the reasoning behind the delay was that Mega Man X4 "had just gotten lost in Sony's back log of games waiting for approval".

[36][37][3][40][41][42][48] In particular, GamePro and Next Generation both gave it negative reviews on the sole basis of its perceived lack of series innovation; GamePro asserted that "the gameplay's none removed from Mega Man for the NES - things are just a little bigger and a little louder,"[48] while Next Generation suggested that those interested in the game should instead "pick up Mega Man X3 in the used bin for a third of the price, since you won't miss much.

"[47] GameSpot took more of a middle ground, concluding that "All in all, a few more 3D effects would have been nice, but the decision to stick with a true 2D environment is bold, if somewhat outmoded.

[36][37][3][40][45][47] John Ricciardi of Electronic Gaming Monthly differed on the last point, saying that the stages are too easy, but added that "the overall experience is definitely a positive one.

"[36] Electronic Gaming Monthly listed the console versions at number 78 on its "100 Best Games of All Time" in the 100th issue of the magazine in 1997, the same issue in which they reviewed the PlayStation version, citing its "significant improvement over X3, with amazingly detailed 2-D graphics, well-balanced (although slightly easy) gameplay and an awesome story with very well-acted animated cut scenes.

"[49] They also listed Mega Man X4 as a runner-up for "Side-Scrolling Game of the Year" (behind Castlevania: Symphony of the Night) in its 1997 Editors' Choice Awards.

[53] In 2002, Capcom re-released the PlayStation version of the game as part of the North American Greatest Hits range, confirming that it had sold at least 350,000 units.

[54] Mega Man X4 has also been re-released in multiple budget versions in Japan including PlayStation the Best, PSone Books, and Sega Saturn Collection.