Bill Rodgers, 65, the greatest American marathon runner ever, plans to come out of retirement to run in the Boston Marathon next year, a race he won four times. “He will prove a point that you don’t mess with runners,” his biographer, Matthew Shepatin, told The Post’s Richard Johnson. “They stand united.” Turnout will be huge next year on the anniversary of the bombing that killed three and wounded dozens. Shepatin’s “Marathon Man” — just out from St. Martin’s — tells the story of likable, shaggy-haired Rodgers, whom sportwriters dubbed “Boston Billy” and played a huge role launching the running boom. Rodgers also won the New York Marathon four times.