The Game's Four Hundred or The Game's "400" was the subtitle of the almanac, the Baseball Register, in the early years of its publishing history that began about 1940.
The Register is a former annual publication of The Sporting News. It listed the career statistics of the current major league baseball players.
The subtitle was derived from "The Four Hundred", the nickname coined by Ward McAllister for the social elite of New York City in the late 19th century, also known as the "Knickerbocracy". The New York group in turn derived its name from The Four Hundred that ruled Athens after the Athenian coup of 411 BC.
At the time the Register began publishing, there were 16 major league teams and 25 players per squad, which coincidentally works out to 400.
The use of that phrase in 1940 indicated the strength of the cultural memory of that ultra-wealthy class of New York citizens.
The phrase was finally abandoned with the 1965 edition, perhaps after it occurred to the editors that the major league expansion to 20 teams several years before, along with the passage of time, had rendered the phrase obsolete, at least in that context.