True, the Pikachu from Brawl and 4 is the Pichu from Melee. Melee Pikachu died after that game.
Pikachu was the top character in Smash 64. He took his job very seriously and was S tier because of it. Ah he was the envy of the entire roster back then. What with his infinitely vertical Thunder combos and quick yet powerful attacks. Melee came along and the yellow Pokemon slowed down a bit, his attacks not as potent and his Thunder not as fearsome. A Pokemon in his prime winding down. This is when the little upstart known as Pichu, much mocked for his constant zapping of himself but his quickness was something to be admired, join the fight. No matter how hard the little baby pokemon tried, despite looking up to his formally S tier peer, he could never quite shake last place on the tier list. Pikachu knew he was getting old and he took pity on the little Pichu. It was no surprise to anyone that Pichu was not invited for the next Smash when Pikachu did something crazy.
Just as Mario was beckoning Pikachu to join the pre-Smash training ring, Pikachu revealed a Thunder Stone, transforming himself into a Raichu. He was disqualified from the fight on the grounds that no one gives a shit about Raichu. So he decided to go from trainee to trainer as he set his sights on Pichu, determined to get him up to snuff before the deadline. Not even Little Mac and Doc Louis could match the grit and determination that was Pichu and his Raichu trainer. The deadline was upon the electric Pokemon and the last hurdle to get into the roster was nigh. Pichu was placed last on the consideration ballot and will fight Little Mac to ensure his spot on the roster. Will this be the first time one of the Starting Eight will not make the fight?
The battle was rough, both Smash hopefuls battered, bruised, and having numerous electrical burns. The cement truck fists of Little Mac proved to be too much for Pichu, and only by the skin of his teeth was he able to latch onto the ledge before being helplessly pummeled right off the edge of the stage. With a tear in his eye he took one last look at Raichu. With merely a nod from his long time Smash hero, Pichu found the last determination he needed to pull his little body back up onto the stage. Little Mac was merely one swing away from charging up his KO move in full. With fierce determination he lunged right at Pichu, almost taking flight as he revved up his punch...when Pichu decided to sleep on the ground. Little Mac flew right over the tiny Pokemon's head, plummeting straight into the abyss with his shitty recovery.
With his victory, Pichu felt strength swell up inside him as his body began to glow, evolving into the most famous Pokemon the world over: Pikachu. With Little Mac officially bumped to Assist Trophy for Super Smash Bros Brawl, Pikachu once again took his place firmly as an unbroken veteran for Smash Brothers. Raichu knew his little pupil surpassed him in skill, going on to become one of the most effective fighters in the soon to be massive roster. He gave Pikachu his favorite green party hat as a parting gift, before officially retiring. With one last brotherly hug did Pikachu send off his long time hero before joining the rest of the Nintendo cast for some good, old fashioned ass kicking fun.
This is my story of Pichu: Zero to Hero! (Hence the explained goggles, wall jumping, and volt tackle)