I've thought about this and I think it would be:
- Ruth (self-explanatory)
- Jackie Robinson (ditto)
- Honus Wagner (the first superstar player in American team sports history, the model of what every baseball hero would follow)
- Willie Mays (the platonic ideal of the complete baseball player)
I get the temptation to put a modern player on Mount Rushmore, but realistically Mount Rushmore is a celebration of history and I think these are the four most important and influential players in the history of the game.