The fourth installment of the Mario Kart series, released in 2003.Changes from the previous installments include, most noticeably, the fact that players must choose two characters per kart, which means that two players can also play as one kart. Also, we are introduced to All-Cup Tour, which is a fifth cup in which every course must be completed in a random order, always beginning with Luigi Circuit and ending with Rainbow Road. This feature has not been brought back in any other game. The game consisted of twenty characters overall, which meant ten pair. Eight karts on the track at all times.
This Game provided examples of the following Tropes:
Boring, but Practical: Koopa Troopa and Koopa Paratroopa's unique items were triple green or red shells. Unlike other games in the series where the triple shells could be obtained by anyone, triple shells were limited only to these two (and King Boo and Petey Piranha). And while certainly that it's not as unique as the rest of the other characters' unique items, they could gain triple shells at around the same frequency as regular shells making them quite a nuisance in races.
Car Fu: Found in the battle modes. This game is also the only one to have "Bob-Omb Battle", which is where karts would attack each other with nothing but Bob-Ombs. However, it was unofficially brought back in Mario Kart 7 through an internet custom group.
Couch Gag: Lakitu has a different item at the end of his fishing rod at the intro of the game.
Easter Egg: If you press the item button when you don't have an item in your possesion, you'll honk the horn. It doesn't do anything other than making your gunner do a pose.
Everything's Better with Rainbows: Of course, nobody really ever seems to like Rainbow Road. It's a love it or hate it. But what really drives you crazy is if you're struggling to maintain the lead in the All-Cup tour, and you've gotta finish higher than the guy in second, and then this comes up as your final course. It always does.
Socialization Bonus: In two player mode only, if both drivers are holding a type of mushroom, and both press the item button at the same time, they will perform the titular Double Dash. The same applies to the Rocket Start at the beginning of each race.
Two players in the same kart also allows the player in the back to perform a swipe by pressing the L or R buttons, depending on the direction. Along with causing racers to spin out, it'll steal items, and can also be used to help reorient the kart.
Take the Wheel: Drivers can actually switch off during races, where one character takes the wheel, and the other handles the items.