The flatback four is a defensive formation in football (soccer) that gains its name from the fact that all four defensemen play in a flat, linear formation, horizontally accross the field. Lacking both a zone or man to man marking system, it is generally used with a 4-4-2, 4-3-3, or even a 4-5-1 formation, the flatback four was the defensive organization of choice for the teams at the recent 2002 World Cup in Japan/South Korea, including the United States' National Soccer Team. It is a fairly common formation in the higher club and school levels, as well as professional and international arenas but can often be difficult to use properly.

The main purpose of the flatback four is to create a very strong, layered defense. How is flat layered? It works like this. If a striker (forward) makes a run down the right side of the field, the right outside defender and the right inside defender will both immediately get the angle on him and double-team him/her until he is forced to give up the ball. The inside left back (defender) will get about at least three yards behind, and 5-10 yards to the left as a third line of defense who will watch for runs in from the midfield, and move onto the ball himself if the striker breaks through. The outside left back waits at about the center of the field to break up any crosses, and mark anybody who makes a run to the back post. If somebody makes a run down the left line, it works the same way only in reverse. If a striker makes a run down the middle whatever ball is being passed to them will go right to the goal keeper, or he/she will be promptly separated from the ball by the two central defenders.

One of the most important features of the flatback four is the ability to pull an "offsides trap." Offsides is when a member of the opposing team, who is taking part in the play (either by moving to a pass, or making a run for a future pass), is behind the last defender when a ball is passed beyond the last defender. The offsides trap is accomplished when all four defenders step up at once causing one or all of the opposing teams' offensive players to become offsides. Since the defensive line is flat, it is often easy to get the other team into the trap as long as every member of the defense steps up at just the right time. Timing the step properly is one of the main difficulties of playing the flatback four. By succeeding in an offsides trap repeatedly though, a defensive line saves itself a lot of running, while frustrating the opposing players. It also allows the defense to win a lot of space quickly and hold it, because if they hold at half field, the other team cannot legally be in their half, and if the opposing player does manage to get the ball, the defense has 30-40 yards in which to chase him/her down while he/she is slowed down by having to dribble the ball. This gives the defense time to manuever the formation to stop the run.