Well, 'quads' involve four players usually, and it's common custom to play two games with each opponent (one of each colour).
The Sit & Go tournaments on this site involve a minimum of 3 or 4 (the Quads), and 5 or 6 (or 10 for the World Warzone) players. It's nice to have events that are already "organized" for you and await merely enough participants before they will start up (you enjoy the convenience of auto-pairing, prizes that mostly return the overall entry fees, a tourney chat room, etc.).
If you have enough friends that are interested at say, a similar rating level, you can pretty much enjoy as many of the "sit & go" tourneys to yourself as you like with just your friends along with a very fair chance at the prizes [so long as you and your friends enter briskly and do not allow an opportunity for a much stronger player or two to enter].
I'm thinking especially about the 960 Crown Stars tourneys [750 cubits entry fees, 10 crowns as prizes, 5 or 6 players needed to start]. It's safest to get 6 to enter as that leaves no room for a latecomer, and starts the event right away.