Batting average on balls in play (BABIP) is one of those leading indicator metrics that can identify potential outlier performances. It is not a perfect gauge, but extremes are rarely sustainable. Pitching BABIPs tend to regress to something near .300. Batting BABIPs tend to regress to a player's own established career rate, which typically does not stray far from .300.
In the history of Major League Baseball, there have been only 26 instances where a player has gone an entire season (minimum 500 plate appearances) with a BABIP of .400. Ty Cobb alone makes up seven of those 26 instances, including the all-time record of .444 in 1911. George Sisler, Rogers Hornsby and Shoeless Joe Jackson each make two appearances on the list.
There are only four players who've accomplished the feat since 1936: Roberto Clemente (.403 in 1967), Rod Carew (.408 in 1977), Manny Ramirez (.403 in 2000) and Jose Hernandez (.404 in 2002).