The SIN corporation tries to analyze Ryu's power "Satsui no Hado" to incorporate it into their mysterious BLECE project with M. Bison being the power behind SIN. It introduces a new system called Focus Attacks, as well as Ultra Moves.
Street Fighter IV takes place several months after the events of Street Fighter II (thus is chronologically set between Street Fighter II and III). After M.Bison's survival from his encounter with Akuma, the S.I.N. corporation organized another fighting tournament in order to draw out the most powerful fighters on Earth in order to compile their fighting data for nefarious purposes.
