One of the fundamental differences between the two systems is who provides the width. In a formation with three centre-backs, that is always the wing-back.
"They are automatically responsible for maintaining the width of the pitch, both in the attacking and defensive thirds. That is extremely demanding. They have to be good enough defensively while also being creative and productive in the final phase, essentially at the level of a winger," explains Flashscore's data specialist Marek Kabat.
With four defenders, the shape is more flexible - the width can be provided by a winger, a full-back, or through a hybrid system. It is also a less demanding option for centre-backs.
"They do not have to defend as many spaces left open behind the wing-backs in a three-man system, and they do not need the athleticism to stop wingers. They are also not expected to progress the ball in the build-up," the analyst compares.
According to him, neither model is inherently more attacking or defensive. However, data shows that the three-at-the-back system is more often used by teams considered weaker on paper or those near the bottom of the table: "It offers many advantages in terms of compactness and defending the penalty area."
Manchester United offered an excellent example of the differences between the two principles last season, with the change brought about by a change of manager. At the start of the calendar year, Ruben Amorim was replaced by Michael Carrick. While the Portuguese coach relied on a three-centre-back formation (3-4-2-1), Carrick changed it immediately after taking over and now uses a 4-2-3-1.
"Amorim inherited a squad that was not suited to his system. For example, Bruno Fernandes had to play as part of a double pivot instead of in the more advanced number 10 role between the lines, which did not make the most of his strengths," Kabat points out.
Kobbie Mainoo, meanwhile, was not physically suited to handling both defensive and transition duties in the same position, so he did not play much, according to Kabat. The full-backs and Lisandro Martinez were not a good fit either.
"Carrick succeeded at the start mainly because he returned the players to their natural positions and allowed them to make the most of their strengths," he explains. Even he is now, in Kabat's view, struggling to find the ideal role for his players, including Patrick Dorgu.
Another major difference between the two systems is how they are used in specific competitions. While the vast majority of Premier League and LaLiga teams favour four defenders, a back three is far more common in Serie A and the Bundesliga. In Central Europe (Czechia, Slovakia and Poland), the two options are evenly balanced.
"The question should not be which approach is better or more popular. It depends on what the coach wants to achieve and which tools and players he has at his disposal," says Kabat.
Premier League managers, in particular, have taken defensive formations a step further. Pep Guardiola did so during his time at Manchester City, teaching Josko Gvardiol to move inside from left-back, a system that was later adopted by others.
Then there are hybrid players such as David Hancko. At Atletico Madrid, he plays in the centre of a back four, while for Slovakia he operates on the left.
"That is because the national team lacks quality in the final third. Apart from Lukas Haraslin, nobody consistently brings European-level quality there. Hancko therefore moves inside from the left into the space between the lines, essentially playing as an eight, while Haraslín provides the width. This combination produced Slovakia's most dangerous moments," Kabat explains.
Nor is there just one way to use a back three. It can be attack-minded, as demonstrated by Xabi Alonso at Leverkusen, or defensive, as practised by Oliver Glasner at Crystal Palace.
"He is an excellent example of how a more rigid system can maximise the potential of a team. He won the FA Cup, Community Shield and Conference League with it. His team sits in a mid-block, relies on defensive compactness and creates chances primarily through quick counter-attacks," the analyst notes.
However, the aforementioned Xabi Alonso is currently struggling to make his approach work at Chelsea, despite its success at Bayer. The Blues have already conceded 13 goals, the most in the Premier League.
"At Leverkusen, Alonso solved it with a hybrid system. On one side, Jeremie Frimpong essentially played as a winger and held the width in the opponent's half, while on the other side Alejandro Grimaldo operated more like a traditional left-back. At Chelsea, he has Palmer and Rogers as two number 10s, which is an ideal use of both players, but the make-up of the wing-backs creates defensive risks," warns Kabat.