For a full season, 3 outfielders are never enough. Fourth outfielders need the defensive chops to play any of the 3 spots and preferably a good bat as they'll get their fair share of starts. It's even better if the 4th outfielder has a complimentary skill set to the rest of the outfield or exceptional defense, allowing him to replace a weaker defensive player in the field as a late game substitution.
Selection Willie Mays
Willie Mays is the greatest centerfielder of all time. He's also the most complete player at any position, the (arguably) second best bat at his position and the second best defender at his position of all time. Not surprisingly, he was originally my starting center fielder, but on further reflection (and watching certain friends' bench selections), I realized that, in the context of this lineup, he was only the second best choice. Still, that made him the automatic choice for fourth outfielder. Willie was a career .302/.384/.557/157 hitter in a 21 year, 2992 game career. Additionally, Fangraphs credits him with 185 runs saved over the average centerfielder. It's worth mentioning that Mays had 7 years with a Wins Above Replacement* (WAR) greater than 10 (deep MVP territory) and a lifetime WAR of 163.2. That's 4th overall, in a practical tie for 3rd with Ty Cobb. Also note worthy, he's the highest ranked right handed batter in terms of WAR. Oh, and he hit 660 home runs and stole over 300 bases. The man knows how to play baseball.
His peak was pretty high, too. His best season was probably 1965, one of his two seasons with more than 50 homeruns. His batting line was .317/.398/.645/187 and he had about 15 runs saved on defense.
*This is important. If you don't know what WAR is, please click the link. Briefly, it's an estimation of a player's contribution to team wins above a replacement level player (roughly an average AA player). WAR is according to Fangraphs.
Wait, Willie Mays is the second best defensive center fielder of all time? Who could be better? It turns out that Andruw Jones had probably the greatest 11 year stretch of defensive dominance of any player ever, averaging 25 runs saved above average** per year. In triple crown stats terms, it's equivalent of adding 21 home runs and 25 RBIs every season without changing his average. Or 75 extra stolen bases.
Jones is also an excellent example of a great player who's batting average is slightly below average. His .256/.338/.488/112 batting line is merely good, but it's definitely above average. Add in the value of his defense, and Jones is clear Hall of Famer, even with his ugly 2008 season with the Dodgers. While he's still putting together strong seasons as a part timer, his best full season was 2005, with a .263/.347/.575/133 batting line and 26 runs saved above average.
Jones was my original pick for 4th outfielder, but he wasn't going to hold the position when Mays was available. If you plan on using your 4th outfielder almost entirely as a defensive replacement, Jones is the pick for you.
** Since runs saved (above average) is in units of runs, we can approximate the value of a player's (above average) defense by adding stolen bases (or converting some singles to homeruns). The advantage of the stolen base methodology is that it doesn't effect the player's batting rate stat and it's an easy conversion. For every run saved, add 3 stolen bases (or 0.85 home runs (not hits, just convert singles to homeruns)). So we can treat May's as league average defender if we credit him with an extra 555 stolen bases (or another 150 or so home runs).
Or several other all time great outfielders with centerfield experience. Musial and Cobb are left handers and a little better with the bat (for their times) than Mays. I tend to discount deadball and pre-integration performances some, so I don't give Cobb quite the respect his .366/.433/.512/171 batting line otherwise deserves. Also, Cobb was a jerk and that's being nice. Musial, baseball's valiant knight, had a more legitimate .331/.417/.559/158 line. He's a good pick if you prefer a left hander for your 4th outfielder. Or if you're such a Dodger zealot that Mays and Jones are anathema to you.