With three or four players the basic rules are the same. The game goes counterclockwise, meaning that you attack the person on your right. Whatever card you throw at them, every other player is free to match. That is, if you’ve given them a six to beat, every other player who has a six is welcome to throw it in as well, and the defendant must beat them all. If they use a seven, once again, everyone is welcome to throw in their sevens (but no more than six cards altogether; that rule still stands). If the defendant can’t beat all the cards, they must accept and skip their turn. The next person to attack will be the player on their right.
As you see, when you have more players it gets more difficult to defend yourself. With four players, it is next to impossible, since every time you’re attacked you are almost guaranteed to have to beat six cards. Unless you are lucky to have a very good hand, you will have to accept. However, you have more chances to get rid of your cards as well: as soon as the next player attacks, you can join them in the attack if you have matching cards.
That’s all you need to know. Good luck to you playing “The Fool!”