Which Street Fighter II update first made the four boss characters playable?

The Street Fighter II update that first made the four boss characters playable was Champion Edition.

Released in arcades in 1992, Champion Edition followed Street Fighter II: The World Warrior. Its headline change was turning the previously computer-only Grand Masters—Balrog, Vega, Sagat, and M. Bison—into selectable fighters. It also introduced mirror matches, allowing both players to choose the same character with different color schemes.

Champion Edition was more than a roster expansion. Capcom rebalanced the original eight fighters, revised several backgrounds and portraits, and adjusted the single-player structure. The four Grand Masters were toned down from their powerful CPU versions, although they remained distinctive and effective fighters.

A common mix-up is choosing Hyper Fighting or Super Street Fighter II. Hyper Fighting came later and focused on speed and new techniques, while Super Street Fighter II added four entirely new characters. Champion Edition is the specific revision that opened the original four bosses to human players.