The game takes place after the Street Fighter II tournament, but before Street Fighter III.
The arcade version of Street Fighter IV features all twelve World Warriors from the original Street Fighter II as part of the regular roster, along with four new characters C. Viper, Rufus, Abel and El Fuerte. The game also introduces a new boss character named Seth. Seth becomes selectable after completing the arcade mode with all of the selectable characters, including Akuma and Gouken.
Two hidden characters are featured in the game. One of them is Akuma, who is both, a hidden challenger, as well as secret selectable character. The other is Gouken, Ryu and Ken's sensei, making his official debut in a fighting game. Gouken appears as a secret challenger, if you are able to defeat him he becomes selectable.
The home versions featured all the characters from the arcade, as well Fei-Long, Dan, Sakura, Rose, Cammy and Gen. Seth can be unlocked as a playable character in the home version of the game, too. Super Street Fighter IV featured returning characters, Dee Jay, T. Hawk, Cody, Guy, Adon, Dudley, Ibuki and Makoto along with two new characters, Juri and Hakan. Yun and Yang rejoin in the arcade version.
Street Fighter IV
- Balrog: After M. Bison was defeated in the second Street Fighter tournament, Balrog gained temporary control of Shadaloo before running the organization into the ground. He joins the new tournament at Bison's request in hopes restoring his wealth.
- Blanka: A mutant who grew up in the jungles of the Amazon in Brazil, after being separated from his mother in a plane crash. He enters the tournament to make his recently reunited mom proud and spare her the embarrassment of having everyone in town staring at them.
- Chun Li: An accomplished member of the International Criminal Police Organization, or Interpol, Guile asks Chun-Li for her assistance investigating a tournament involving Shadaloo. No doubt, she also hopes to extract further revenge for the murder of her father from M. Bison.
- Dhalsim: Although Dhalsim was prepared to put his street fighting days behind him, he could not ignore the knowledge that his abilities might help the forces of good triumph against a potential great evil known as S.I.N.
- E. Honda: Invited as a past attendee of previous Street Fighter tournaments, E. Honda is one again anxious to prove to the world that sumo wrestling is more formidable than most people give it credit for.
- Guile: Unable to accept the fact that his friend is truly gone, Guile is about to resign from the US Air Force and search for him on his own when his commander assigns him to a new mission involving a possible connection with Shadaloo.
- Ken Masters: Although excited by the prospect of entering another Street Fighter tournament and getting the chance to spar with his close friend and only true rival, Ryu, Ken is unsure if he should leave the side of his pregnant wife until she reassures and encourages him.
- M. Bison: Seeing himself as the rightful dictator of the world, M. Bison appreciates the power of the human fighting spirit, and technology's ability to enhance it, over firearms and weapons of mass destruction. After his body was destroyed in the second Street Fighter tournament, M. Bison returns with a new body in an effort to co-opt the tournament to further his own diabolical plans.
- Ryu: Unable to turn down any opportunity for a good fight, Ryu eagerly enters the next Street Fighter tournament for a chance to train harder and expand his fighting abilities beyond all limits. But could this tournament be an elaborate trap to capture this fabled fighter?
- Sagat: It's been many years since Sagat's defeat at the hands of Ryu. Sagat realizes that there is more to fighting than just winning, but he is still striving to figure out what that is. Perhaps he will find the answer in the next tournament.
- Vega: Never quite content with the boring aristocratic life he returned to after the fall of Shadaloo, Vega sees the creation of a new tournament as a chance to immortalize not only the original organization, but his youth and good looks as well.
- Zangief: Having already proven his greatness to his home country once before, Zangief is prepared to repeat the feat in the latest tournament. And this time, he's not just doing it for home and country, he's doing it for a new young friend as well.
- Abel: Abel is a mysterious fighter from France with no memory of who he is or where he came from. He enters the tournament with the hopes of finding clues about his past.
- Crimson Viper: Crimson Viper is a skillful mercenary who has been hired by the people behind the new tournament to gather data on potential street fighters. She fights using a prototype suit that enhances her fighting abilities with flame and electricity.
- El Fuerte: El Fuerte is a Mexican fighter with two passions: traditional Mexican wrestling and cooking. He seeks the exhilarating thrill of giving everything he has and improving his talents at both.
- Rufus: Reviled by the fact that Ken Masters is considered the greatest fighter in America, portly Rufus enters the tournament determined to prove once and for all that it is really he who is best. And if he gets a chance to kick Ken's butt on national TV, so much the better.
- Akuma: Akuma continues to seek only those opponents worthy of facing his indomitable might. Sensing that Ryu may be on the verge of achieving Satsui no Hadou once again, he enters the tournament with the hopes of having his wish for the ultimate battle fulfilled.
- Gouken: Thought to have been long dead, the master of Ryu and Ken, and the brother of Akuma, mysteriously reappears and shows up at the latest tournament with an unspoken goal: to seal one's awakening chi with the Power of Nothingness before it is too late.
- Seth: A creation of the S.I.N. organization, a synthetic being, Seth was originally known as Number 15 until he gained greater self-awareness and overthrew his creators. Continuing on with their plan, he intends to succeed where M. Bison has failed.
Console versions only
- Cammy: Confident in her abilities as a fighter and as a member of the Delta Reds, Cammy is still haunted by her past as a living doll assassin for Shadoloo. Cammy suspects that she may find clues about her origin in this new tournament.
- Dan: When Dan hears about the latest Street Fighter tournament, he is sure his invitation was lost in the mail, and shows up as always to prove that his personal style of martial arts is unbeatable.
- Fei Long: Fei Long had not planned on attending the latest tournament… until his movie crew was unexpectedly attacked by a gang who left the calling card of S.I.N. Angered that someone would dare interrupt his filming, he vows to get revenge.
- Gen: Old and tired, Gen investigates the events that surround the latest Street Fighting tournament. Wishing to die only in a great battle, he seeks eternal rest at the culmination of exhilarating combat.
- Rose: Never quite sure what took place immediately after the events in Street Fighter Alpha 3, Rose is determined to attend the latest tournament and complete a job that she feels was left unfinished.
- Sakura: Wishing for nothing more than another opportunity to find and possibly spar against her hero and inspiration Ryu, Sakura finds out about the new tournament from Dan. Nothing can keep her from finding out if Ryu will enter.
Super Street Fighter IV
In addition to the characters above, the following will be available in Super Street Fighter IV.
- Adon: Adon no longer sees Sagat as the Emperor of Muay Thai. Disgusted by the thought of Sagat going soft, he plans to dethrown him and end his ways for good.
- Cody: Cody, the fallen hero of Metro City, breaks out jail once again. Looking for something to curb his boredom, he follows his old friend Guy, to the tournament.
- Dee Jay: Dee Jay learns of the new world tournament, not being challenged by bar fights he enters to have some fun and show the world his unique style of rhythm.
- Dudley: Dudley is invited to the international fighting tournament. Despite the invitation lacking in social graces, Dudley enters intending to win the prize money to get new roses for his garden and provide a distraction for his missing car.
- Guy: Tired of watching evil have its way on the city and around the world, Guy seeks to use the power of Bushin-ryu sets out to restore the peace.
- Ibuki: Bored with ninja training Ibuki has grown curious of the outside life, especially about boys. She ask her friend to cover for her in training camp while she has a little fun.
- Makoto: Makoto enters the current tournament to win the prize money. With it she can repair the dilapidated Rindoukan karate dojo, that once belonged to her late father.
- T.Hawk: Not feeling that he deserved the title of chief of the Thunderfoot tribe, T. Hawk enters the tournament to find some information on his lost love, Julia.
- Juri: Juri, Seth's henchwoman, forces him to give her a more challenging assignment, or else she would make her own fun. Seth then enters her into the world tournament, where she has her own agenda.
- Hakan: Hakan, the "Oil King of Turkey" heads to the world tournament. He's determined to show the world the superiority of Turkish Wrestling, and the usefulness of his oil.
Arcade version only
- Evil Ryu: Succumbing to the dark power of the Satsui no Hadou, Ryu seeks opponentsto quench his thirst for battle.
- Oni: Losing all his humanity, Akuma becomes Oni, "evil incarnate".
- Yang: Wanting to keep an eye on his impulsive twin Yun, Yang tags along with him as he tails Chun-Li. His curiosity piqued about who Chun-Li is going to fight, Yang wants to test his skills against strong warriors.
- Yun: Curious about where Chun-Li was headed to, Yun along with his twin brother Yang follows her to the tournament. He ensures Yang that he only wants to know what Chun-Li's doing, but is ready to fight "if someone starts something".