SMG4: Stupid Mario Tennis Aces is a blooper uploaded on July 7th, 2018. This is also serves as a prologue of The Waluigi Arc.
The video begins with a match of tennis between Luigi and Waluigi that resulted in Waluigi winning, but to Fishy Boopkins it seemed very funny. He then asked to play, but Jeeves, the referee, said that the two were training for Tennis Aces, and that he should first train in order to be the best. However he allowed Boopkins to play, only that Boopkins must win the match in order to be accepted.
Boopkins claimed to be a pro player in tennis, but misses against Waluigi, whose ball hits him in the face. Leaving the tennis match, Fishy Boopkins cried all the way back home, until Mario appeared in a wheelchair due to an accident from playing tennis with a Chain Chomp. Boopkins believed that he was Naruto before he revealed himself, seeing Boopkins weeping over his prior loss. Mario then offered to train Fishy Boopkins to become a professional tennis player, so that the money that they earn from winning Tennis Aces could help him pay bills for a hospital treatment.
The training session began with Fishy Boopkins attempting to throw his ball, only that it just dropped straight down to the ground. Mario then told Boopkins to use his mouth, not knowing whether that would violate the tennis rules or not. Boopkins did what he was told, and he was able to launch the ball high enough to smack it with his racket, albeit hitting and killing an old man on wheelchair in the process.
Up next, Mario decided to launch a series of tennis balls at Fishy Boopkins to test how efficiently he can hit the ball back at him. Boopkins was able to hit all the balls hurled at him, until Mario started to turn up the difficulty by throwing other various objects at him, such as a chainsaw, melon, Baldi, and even a fighter jet. Fortunately, Boopkins was able to knock Luigi back at Mario when the former was put in the launcher, sending Luigi to outer space.
After Luigi was hit out of the ballpark, Mario told Fishy Boopkins about a "special move" that he may have in tennis. Although Boopkins didn't know what his move was, Mario just advised him to believe before launching many tennis balls at him simultaneously. Eventually, Boopkins' racket turned into his "waifu" body pillow and was used to deflect all of the tennis balls from the launcher. Impressed by Fishy Boopkins' performance, Mario decided that he was ready to have a rematch with Waluigi at Tennis Aces.
Waluigi returned to have a match against Luigi, who kept losing to him. But Fishy Boopkins then surprised him with a request for a rematch, defeating Waluigi once with his newfound tennis skills. Jeeves decided that there was only one way to settle the match, by having the first player to earn one point win.
In the match, Boopkins played against Waluigi and held off quite well, until he lost his racket when Waluigi used his Waluigi Launcher attack to hit the ball. Out of options to hit the incoming ball, Boopkins then used Mario's sign as a temporary racket, before throwing Mario at Waluigi and recovering his own racket as the match became very intense. Waluigi uses his special move, which was simply a taco stand attack. But at the same time Waluigi used his move, Fishy Boopkins used his special move in retaliation, hitting the ball with his "waifu" and sending Waluigi flying around the world (Including Inkopolis) before KOing him. Boopkins thought that he won upon seeing Waluigi burning on his side of the court, but Jeeves stated that since the ball didn't touch the ground once, Fishy Boopkins had committed a foul with the win still going to Waluigi.
Waluigi went to celebrate his victory, but then felt guilty when he overheard Boopkins crying about being rejected. He proceeded to convince Jeeves to let Boopkins join the tournament, saying that he knew the feeling of rejection "all too well" (referencing the meme that Waluigi is always rejected from Super Smash Bros. series) and started to cry about being left out. Jeeves let Boopkins join the tournament, and Boopkins took Waluigi to get ice cream in order to cheer him up. As the two walked away, Mario tried to convince Luigi to pay his hospital bills, but he refused after Mario told him about lending one coin earlier.