Defensive block of 4
In my opinion the secret to Southamptons success YES they have switched the system in games at times to suit certain games situation but this has been their default defensive and attacking shape all season.
Here to the left we see the defensive block of four which include the foursome Wanyama, Schneiderlin, Yoshida and Fonte. They all have been the base of the success for Southampton. The strength of this block is that it provides the centre halves with that all needed protection against those tricky players in midfield and those coming in of the wing.This block allows one DM to pressure the player on the ball and lets the other recover the space. They are also the catalyst for the build up play and make use of those quick transitions.
Defensive block of 3
As opposed to the defensive block of 3 which most teams use only allows for one player in front which in the modern game of quick transitions can still be easily overloaded and countered. This block of 3 is still effective because in the defensive transition it allows enough time for the full backs and the midfield to get back into position and provide the defensive block. Unlike with the block of 4 the DM hasn't got as much freedom and cant go and pressure the player on the ball due to his duties of protecting the two centre backs.
Attacking in a Defensive block of 4
At home Southampton tend to play more possession based game aimed at retention of the ball. The two DM's will take there time to push up into the midfield to play with the advanced midfielders. You may think the cautious thinking of the Holding midfielders hold the team back in terms of creativity and expression but Schneiderlin in particular has immense talent on the ball and his ability to create from deep , spot a pass and execute it is a major asset to this team. Also Wanyama has bagged a few long shot goals this season again proving there worth as an attacking threat. Koeman also has the option if he may to go to a 4-3-3 if he needs an extra player higher up the pitch but for that defensive structure 4-2-3-1 is the best way to go.
Defending in a Defensive block of 4
The two holding midfielders never go beyond the ball they always stay behind the ball to a make it easier to recover there position. They never press too high up in case they are caught out of position. The way Southampton play they rely on quick transitions so by having that extra protection in front of the defensive it also the midfield and fullbacks to do more damage going forward rather than have to overly worry about the defensive stability.
Southampton have proved that working in a tight defensive unit and taking advantage of quick transitions that any team no matter the stature can achieve success in football through teamwork, hardwork, determination,tactical ability and a little bit of talent.