Which Mega Man game introduced the Mega Buster charge shot?

Mega Man 4 introduced the Mega Buster charge shot. Its upgraded arm cannon lets Mega Man hold the fire button, build energy, and release a much more powerful blast.

The feature debuted in the 1991 NES game Mega Man 4, where the weapon was called the New Mega Buster. It could be charged to multiple levels, giving players a stronger alternative to the regular pellet shots used in the first three numbered NES games. The charge shot also helped Mega Man damage enemies more efficiently and could pass through defeated enemies in some situations.

Players sometimes confuse Mega Man 4's innovation with Atomic Fire from Mega Man 2, which could also be charged. Atomic Fire was a special weapon with a charge-like function, however; Mega Man 4 was the first classic-series game to give the standard Mega Buster a charge shot. The mechanic became a recurring part of later games, although Mega Man 9 and Mega Man 10 temporarily omitted it.