Since a multiplayer Johnny's race seems... well, it's the same as nothing to me.
The wi-fi with four players could be for cooperative gameplay, each one controlling a character. But then, it would go against Gaspar's bullshit about only 3 people being capable of going through a time gate without going to the End of Time. Unless This DS version made it possible to control characters separated, even in different eras, but I don't believe they would change the game to this depth. Also, battles would be too strange, including double and triple techs.
However, versus mode ain't an impossibility, right?