Baseball teams can be obsessed with maximizing assets. That's why it can almost be a foregone conclusion for a team to trade a good, young catcher if it already has a firmly entrenched starter (like Wilson Ramos and the Twins in 2010). The same principle applies to outfielders who promise to be league average or better -- they rarely warm benches. But the best fourth outfielders can still be important members of a major league team.

First, let me be clear about what I mean by "fourth outfielder." I don't mean outfielders whose playing time is only reduced because of the presence of other players (as with Andre Ethier) or because health makes playing every day impossible (as may be the case with Cody Ross in 2014). Because their teams are still likely to carry five (other) outfielders, I also don't mean players who, despite not starting most games, get a lot of playing time in the infield, as well as the outfield (think Skip Schumaker, or even Ryan Doumit). And since I'm making up my own rules here, I'm also excluding players who, despite being fourth on the team's OF depth chart, are likely to start most games at DH (like Alfonso Soriano).

As for who I do mean, I think there are three criteria that I think fit most good fourth outfielders, after reviewing the current major league rosters:

Some usefulness offensively. There are some outfielders who stick on rosters despite woeful hitting ability, but they're mostly fifth outfielder types with skill sets similar to utility infielders -- a good fourth outfielder can do some damage at the plate. Generally speaking, I think a good fourth outfielder should be able to hit enough for a team to be comfortable starting him most games during a first-string outfielder's 15-day DL stint.

Positional flexibility. Every roster is different, and in reality, some teams do feature fourth outfielders that can only play the corners. That's the exception and not the rule, however, and that kind of arrangement tends to happen more when one of the corner outfield starters can switch to center if needed (like Gerardo Parra or Shane Victorino). It's very rare for a team to carry only four outfielders, even for short stretches, but the best fourth outfielders can, in my view, play all three outfield positions.

A little something extra. This is where things get interesting. It's a long season, and it's a complicated game. So long as they can do everything adequately, fourth outfielders get no extra credit for being well-rounded. A good fourth outfielder is "well oblong," standing out with special skills that, when deployed at particularly useful times, can help a baseball team win a baseball game.

Four Kinds of Fourth Outfielders

Because the special sauce that can make a fourth outfielder great at his job can come in several flavors, the best fourth outfielders come in several types. In keeping with today's four-is-the-flexiblest-number theme, I present: four kinds of good fourth outfielders.

Defensive Standouts: Being able to play three outfield positions without embarrassing oneself is not the same thing as being able to play them well. Some fourth outfielders have the supple wrists to be defense wizards. That can make them great late-game defensive replacements, especially on National League squads, when subbing them in can have another beneficial effect through double switches. If I had been writing this a year ago, Gerardo Parra might have been my pick here. In posting a 4.6 fWAR in 156 games despite creating runs at a slightly below-average clip (96 wRC+) and terrible baserunning (-0.9 BsR), Parra may have busted some notions about the usefulness of defense-first corner outfielders (although above-average in CF, Parra tabbed 41 DRS in 2013, in part because playing most days in RF made somewhat better use of his throwing arm).

Alas, Parra will have to wait for another award (possibly one through arbitration), because it's hard to consider him a fourth outfielder now. For the same reason, I'll exclude Juan Lagares and Peter Bourjos (we may be entering a golden age for defense-first starting outfielders, which would depress offense in two ways -- but that's a different topic). Instead, I think the prize goes to the Giants' Gregor Blanco, whose outstanding defense in 2013 (16.8 UZR/150) was in line with previous performance (19.4 UZR/150 in 2012). Blanco's defense helped him reach 2.8 fWAR in 2013 despite starting just 113 games (although league-average offense didn't hurt, either).

Base Runners: If you don't think a pinch runner can change much, I'd urge you to ask a Red Sox fan about Dave Roberts. Unlike with some of the other flavors of fourth outfielders, skill on the basepaths may not be able to make up for shortcomings in other skills valued for fourth outfielders. Still, it's clear that baserunning skills have value -- Rajai Davis just signed a $10M, 2-year deal despite subpar offense (88 wRC+ in 2013, 87 wRC+ career) and subpar defense (-3.4 UZR/150 in 2013, -3.1 UZR/150 career). Despite starting most of the games he's played in recently (108 G, 75 GS in 2013), I feel compelled to credit Davis's contract to his baserunning. His 10.2 BsR was achieved with just 360 PA, and while that wasn't tops among all outfielders in 2013, only six have topped that mark in the last five seasons (Michael Bourn 13.2, 2011; Juan Pierre 12.4, 2010; Bourn 12.0, 2009; Mike Trout 12.0, 2012; Jacoby Ellsbury 11.4, 2013; Carl Crawford 10.9, 2010).

It would be tough for a part-timer to match Davis in 2014, and so I think he takes the cake in this category. Still, keep an eye on Anthony Gose, who may take over for Davis in Toronto, and on Jarrod Dyson, whose performance in 2013 may have prompted Kansas City to cash in David Lough. Dyson came in 9th in BsR in 2013 among outfielders, despite the lowest PA total (239) of any of the top 50 in that category.

Platoon Splitters: Imagine you were a baseball player visited by a baseball genie when you were 18 years old. He gives you the choice between two gifts: the ability to consistently hit .250 in the majors against all pitchers, regardless of handedness, or the ability to hit .250 overall, with a significant platoon split of .200 and .300. If your priority was just to have a major league career as a player, you might do best pick the latter, because as a fourth outfielder, you might stick in the majors for a decade. And if you're a lefty, you'd be an idiot not to pick the split skill -- as the Cubs proved with Nate Schierholtz in 2013, it's possible to get a lefty hitter more than 500 PA without more than 13% coming against southpaws.

Hitters with significant platoon splits are probably the classic type of fourth outfielder. Some, like Ryan Ryburn (.308/.243), may not actually get used disproportionately; others that do may have splits that are not very pronounced, like Seth Smith (.258/.235). But trust me, they're there, and when a team already has three outfielders who are above-average offensively, a platoon splitter can still get a lot of at bats, like Jose Tabata in 2013 (341 PA, .292/.250). Lefty hitters are probably the best examples of platoon splitters, and so even though it means stretching the definition of "fourth outfielder" (thanks, Ben Zobrist), I'll go with Matt Joyce here, who started 92 games in the field in 2013. Joyce did soak up some DH time (22 games), and he hasn't played in CF since 2009, but with a gorgeous platoon split (.246/.164 in 2013; .260/.194 career), he's a prime example of what a platoon splitter is all about.

Big Boppers: Defense be damned, if an outfielder is a truly great hitter, he's going to play nearly every day (even if it's at DH). I may be struggling a bit to turn this into a category distinct from platoon advantage guys, but I do think there are some fourth outfielders whose playing time can be largely attributed to a better than average ability to put a ball in the cheap seats when called upon for a plate appearance. Overall, there were 4661 HR hit in the 2013 season in 166070 AB -- that's about 36 AB per HR. A 600 AB guy will get some back slaps for hitting 20 HR (30 AB/HR), so I think we can use that as a baseline for who might be a cut above in the HR department.

It will probably surprise no one if I say that a lot of fourth outfielders who hit HRs at a clip better than one per 30 PA tend to be a bit more challenged defensively, and so it shouldn't surprise anyone that fourth outfielders in this category tend to get their outfield time in the corners. If you didn't want to consider guys like Darin Ruf (18 AB/HR) or Evan Gattis (17 AB/HR), I wouldn't blame you. It's probably the case that these guys fit better on American League teams, like Mike Carp (24 AB/HR), but there's definitely a lot of overlap with platoon splitters, including Schierholtz (22 AB/HR) and Joyce (23 AB/HR). Although we only have small major league samples, I'll tip my hat here to Scott Van Slyke (21 AB/HR at AAA and NL combined in 2012; 18 AB/HR combined in 2013).

The Best Fourth Outfielder in Baseball

Like a blank in Scrabble, the very best fourth outfielders in the game will probably be able to check off more than one of the above boxes. So for best fourth outfielder in baseball, I'd like to nominate new Athletics acquisition Craig Gentry. Those that frequently tune into the Beyond the Box Score channel can guess that we pay particular attention to unconventional moves by Billy Beane & Co., and the Gentry acquisition could fit the mold.

Consider that in the same offseason in which they acquired Gentry, the Athletics also traded Seth Smith. Smith doesn't have much of a platoon split to take advantage of, and with a middling BsR (1.7 in 2013), not much power (46 AB/HR in 2013, 28 AB/HR career), and roughly average defense (5.4 UZR/150 in 2013, but 1.8 UZR/150 career), Smith doesn't have any special sauce that might make him particularly useful in particular situations.

With Gentry, who has even less of a platoon split than Smith, I'm reminded that a player need not have a platoon split of his own for a team to get a platoon advantage out of him. Context like Coco Crisp's recent struggles against lefties (.216 in 2013, versus .286 against RHP) does matter, just like offense-first fourth outfielders can be fits for teams that start two outfielders capable of playing center.

Gentry fits our fourth outfielder definition perfectly -- he's more than adequate with the bat (108 wRC+ in 2013, 96 wRC+ career), and he can play all three outfield positions. What sets him apart, however, is that he's got two varieties of special sauce -- very good baserunning (5.8 BsR in 2013) and superb defense (32.1 UZR/150 in 2013, 29.3 UZR/150 career).

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All statistics courtesy of FanGraphs and Baseball-Reference.

Ryan P. Morrison is a Beyond the Box Score contributor, and co-author of Inside the 'Zona, a site on the Arizona Diamondbacks with a sabermetrics slant. You can follow him on Twitter: @InsidetheZona.