Small pictures are clearer.

The elements of soccer have been identified.  2v2, 3v2, 5v2 and so on can all be soccer or soccer like.  However 4v4 is the smallest manifestation of a real match.  In it are all of the elements necessary for players to experience real soccer without any confusing duplication.

In a real soccer match players have the option of passing the ball forward (through), wide or backwards (trail).  3v3 cannot do this because one of the directions will be missing.  While 3v3 is a valuable tool for young children and learning technique, it is limited in its use as a tool for the positional game.  4v4 also provides the minimum numbers needed for all of the principles of play.  One player is up top for penetration.  Two are needed for width and one holds back to supply depth.   Three cannot do this; one of the principles will be left out.  In 4v4 the responsibilities are very clear.  All tasks are covered and none are shared which keeps things simple.

Most importantly, small-sided games allow the players more touches on the ball.  It also puts the player in a greater percentage of coaching moments.  That means, not only does she have to deal physically with the ball more often but she's also involved more mentally.  Her concentration must be greater because the action will be quicker; there is nowhere to hide.  This is an important element in developing the playing mentality.

Team Defense

Soccer games are decided by goals.  Practice centers on creating and finishing opportunities, or, on denying them.  The entire soccer world revolves around one team trying to put the ball in the net while the other tries to stop them.  Yet, when pressed, many coaches and most players have a hard time analyzing how a goal was scored beyond the last touch of the ball.

Here is where small-sided games, along with clear functions, can help. In 11v11 or 8v8 the picture can be crowded, chaotic.  The real reason that a goal was scored might get lost.  In 4v4 the reduced picture makes it easier to read the actual problems.  And since it is real soccer, it bears a direct relationship to the bigger game.

There are, in fact, five basic fundamental reasons why goals are scored. Sometimes, of course, there is a combination of factors, but it is a combination of two or more of the five basic factors.

1) Lack of pressure on the person with the ball.  The forward that breaks through for an uncontested shot.  The midfielder with enough space to play the ball forward to the striker.  Any player that can play the ball forward poses problems for the defense.

2) Lack of support for the pressurizing player.  Attackers look for 1v1 opportunities in the attacking third.  Here the return is worth the risk.  Defenders try to minimize these situations.  They try to keep enough players behind the ball so that when the first defender is beaten another is close enough to control the damage.

3) Giving the ball away.  The careless square pass in midfield.  The lost throw-in.  With so many players placing so little value on ball possession it's not hard to see this as a major factor in goal scoring.

4) Restarts.  This is an opportunity for the attackers to use a rehearsed play.  It is also impossible for the defenders to put pressure directly on the player with the ball.

5) Failure to track players down.  When opponent's run at the defense late and from deep positions the defenders are faced with a significant soccer problem.

Note that reasons 1, 2 and 5 are directly related to a player being in the wrong position.  The player is either too close, too far or at the wrong angle to the opponent.  Reason 4 is partly because direct pressure cannot be applied to the ball.  In all 3 out of 5 reasons why goals are scored are because a player was not at the correct place at the proper moment.  This is often more of a breakdown in the decision making process then in execution of any technique.  When players understand what poor positioning looks like and it's effects they can take advantage of their opponent's mistakes as well as avoid errors for themselves.  They can make a greater contribution to the game.