Well... This idea has been suggested and so on, but the problem isn't having 3-4 players play at once its a lot more than that. The problem is how they will be lined up. Like lets say you wanted to play a game 4 players in aikido. One player would be already disqualified.
When you say
one thing I also thought up was teams
you may want to think of how you want them lined up. I always thought that it would be possible if you had Team 1 and Team 2. Both teams would have 2 players on each team. Now lets say you both want to play at the same time with those teams. I think of it as 2 different dojos and you play regularly.
3 or 4 players may have some thinking and that's the only problem with this suggestion.
It builds up time for this to happen. I like the idea, but think about it.