A team can be described as playing with a back four and still spend most of its build-up phase with three players in the first line. The shape changes with possession because the problem changes with possession.
Build-up is a numbers problem before it is a shape problem
The first line needs enough players to outnumber the opposition's first line of pressure. Against a single striker, two defenders suffice; against two forwards, three are required to guarantee a free man.
Because opponents press with different numbers, the number of players a team needs in the first line varies from match to match, and sometimes within a match.
Keeping four defenders back against one striker wastes players who would be more useful further up, which is why the shape adjusts rather than staying fixed.
The dropping pivot is the most common mechanism
A holding midfielder stepping between or beside the centre-backs converts a back four into a back three plus an advanced full-back on each side.
The advantage is that the extra body arrives facing forward and is usually the team's best passer under pressure, which is exactly the profile the first line needs.
The cost is that the midfield is now one player lighter, so the movement only works if the opponent's press commits players who then cannot cover that vacated area.
Inverted and advancing full-backs achieve the same thing differently
A full-back stepping into midfield leaves three defenders behind him without requiring anyone to drop, which preserves midfield numbers rather than reducing them.
A full-back pushing high up the touchline does the same to the back line but adds width instead of central presence, which suits teams that attack through crosses.
The choice depends on where the team wants its spare player. Neither is inherently better; they solve the same arithmetic in different zones.
Goalkeepers are counted in the sum
A goalkeeper comfortable receiving and passing under pressure functions as an additional outfield player during build-up, which changes the required number of defenders again.
With that option available, two centre-backs can split wide and still hold a numerical advantage against two pressing forwards, because the keeper is the third passer.
The risk is obvious and is why the arrangement demands a goalkeeper who can be trusted with the ball at his feet under genuine pressure rather than in training.
Rest defence sets the limit on all of it
Every player removed from the first line to join the attack is a player unavailable if possession is lost, and build-up structures are ultimately constrained by that.
Teams facing quick counter-attacking opponents keep more players in the first line than the pressing numbers strictly require, buying insurance rather than efficiency.
Reading a team's build-up shape therefore tells you as much about who they are playing against as about what they believe in.