Every year, MLB teams continue to push the limit of what you can do with your defensive alignments. First it was standard infields shifts, and over time those got more and more extreme to the point where you now have players moving after each pitch, and they have been effective enough that the MLB is considering banning them (they shouldn’t). Teams have continued to update and improve infield shifts, and now there are more outfield shifts being explored and employed. The one that has been getting a lot of attention, especially with the Rays going deep in the playoffs, is the four-man outfield. The goal of most infield shifts seems to be turning as many hits into outs as possible, four-man outfields however might not be the same.
The first noted use of the four-man outfield came in 2016, when it was employed twice in the entire season. This year there was an all-time high in usage per game, even though it was really only utilized on a relatively larger scale by three teams, the Rays, the Padres, and the Pirates, and the Rays used it about twice as many times as anyone else. Even though these shifts are becoming more and more prevalent, it is important to know they are still being used in .2% of at-bats.
There are a couple of reasons that teams aren’t able to use it as much as other shifts. First, it is very dependent on baserunners. Of the 115 times a four-man outfield was used this year, 112 of them were with the bases empty, and the other three times it was used during blowout games. It is definitely tough to keep track of baserunners, especially if you have to hold them on, and align infielders in the best possible way to field enough ground balls for the tradeoff to be worth it.
Another reason this extreme shift is very situationally dependent is because without the right kind of pitcher and hitter it seems destined to fail. There are more fly ball hitters now than any other time in baseball history. The way to beat a four-man outfield seems to be by spraying ground balls all around the field, because three infielders can only cover so much ground (including a first basemen who has to stay relatively close to the bag). As a result, teams have really only used these shifts when they have fly ball pitchers in the game pitching to hitters who tend to hit the ball in the air and pull the ball when they hit it on the ground.
The pitchers who have four-man outfields utilized the most behind them really drive this point home. All four of the pitchers the Rays use it the most for (Nick Anderson, Trevor Richards, John Curtiss and Charlie Morton) were all above league average in fly ball rate. In comparison, two of the Rays top ground ball percentage guys (Diego Castillo and Ryan Thompson) combined to use the four-man outfield just twice. On a similar note, the hitter who saw the four-man shift the most in the MLB this year, Cavan Biggio, has had a fly ball percentage of over 30% in both of his MLB seasons (leave average is 22%), and a career opposite field percentage under 20%. He is the perfect hitter to do this against because you can really use three infielders against him. He is unlikely to hit the ball on the ground, and even if he does, it is very likely to be pulled, and you can position guys for that.
You can tell why there are not a lot of situations when teams can use this shift. There has to be nobody on base, a fly ball pitcher on the mound, and a hitter who hits a lot of fly balls and pulls the ball at the plate. Considering that only one team used it consistently and only three teams used it at at all, this defensive alignment definitely has room to grow.
By looking at the matchups where a four-man outfield occurred we can tell what matchups teams like to use this shift for, but we have to go a step further to figure out why they use them. At first, I thought it would seem as if the reason to employ the shift would be to take away hits, like most other shifts, but this doesn’t seem to be the case. Batting average on balls in play (BABIP) is higher against four-man outfields than regular outfields and shifted three-man outfields. This makes sense, because of all the extra space for ground balls to get through. The real purpose of the four-man outfield seems to be preventing extra base hits (XBH). This year there was only one ball that was put in play (excluding home runs) that went for extra bases. Isolated power (ISO), which is calculated by subtracting batting average from slugging percentage, was far lower against four-man outfields than against any other defensive formations. It appears as if teams are willing to accept a marginally higher BABIP against them in exchange for the balls in the gap and down the lines that are prevented from getting to the wall. As a result, weighted on base average (wOBA) was also lowest for four-man outfields.
The sample size for all this is still very small, so it would be foolish to make conclusions about what exactly the purpose of this shift is, but so far, the data supports using it. There are obvious limitations for how often teams will be able to use something like this, but I wouldn’t be surprised to see the number of teams doing it to go up again next year. After all, it seems like every time the Rays are willing to go out on a limb to try something new it doesn’t take too long for everyone else to do it as well.