Mega Man 4 — The Charge Shot Arrives
One new mechanic quietly reshaped the whole series.
By Mega Man 4 (1991), the formula was set, so Capcom added the one thing that would define every game after: the chargeable Mega Buster. Hold the button, release a big shot. Suddenly your default weapon mattered.
It also played with the story: this time the villain appears to be the kindly Dr. Cossack, before the real mastermind (you can guess who) is revealed in a twist. You even get the helpers Eddie and Beat across this stretch of the series.
Some call it the start of the series coasting; the charge shot says otherwise. It's the entry that made Mega Man feel powerful.