The 4231 is an interesting system. It can be so easy for this system to go belly up, and it gives me the perfect platform to answer someones queries about balance.
Whats the most important thing about football? Control of space, and thats it. The whole idea is to make space a premium, if you can make it hard for the opposing team to move the ball around, you give yourselves a strong chance of dictating the game.
Here in the system I have outlined, why have I used 2 holding midifielders? If control of space is important, then it follows that the most important area of the pitch is the middle of the park. If you lose control of the middle of the park you make it very hard to build anything or have any successful transitions.
The 4231 only has 2 midfielders, when its facing a 451, a 442 a 4132 or a 4312 its going to have its work cut out in midfield. The lack of numbers a 4231 has in midfield is its biggest weakness. So when I see people stick a support/defend or a defend/attack combination I immediately know it has issues. Those 2 midfielders are the only protection for the 2 central defenders and the fullbacks. You always want to control space, and if you have one player defending that area of the pitch, then whenever the opposition overloads you will struggle. Any attack where there is more than 1 player on that side of the pitch will render your system defenseless.
When people think in terms of pairs and combinations, one needs to remember that pairs and combinations also exist in the most broadest sense, how the defense and attack combines to work together. By putting 2 players on defend duty, I know that my backline will now have adequate cover since the midfield screen won’t go missing. Furthermore the support roles for my fullbacks ensures that the midfield and defense are working together as one unit, this will leave me free to smash the flanks up if i choose.
So when setting up your own system, you need to consider the overall shape of your own tactic and how components combine together to control space. I can now even switch the inside forward on the right into an attack duty if I sniff a weakness on that flank. Something I did against Benfica in the European Champions League, I think its called BusttheNet – Hudder and Exploit.
Exploiting the AI and looking for weaknesses is not hard, but one needs to have a midfield and defense that works as one unit, a combination that locks down and defends as a unit. And this is only really possible when you have players that provide an effective screen in midfield, this gives the defense a chance to get into position and provide the line of cover. If you have midfielders moving up to attack, the screen doesn’t exist and you will see pressures exerted on your defenders.