The claim that a 4-minute mile was once thought to be impossible by informed observers was and is a widely propagated myth created by sportswriters and debunked

The claim that a 4-minute mile was once thought to be impossible by informed observers was and is a widely propagated myth created by sportswriters and debunked by Bannister himself in his memoir, The Four Minute Mile (1955). The reason the myth took hold was that four minutes was a nice round number which was slightly better (1.4 seconds) than the world record for nine years, longer than it probably otherwise would have been because of the effect of World War II in interrupting athletic progress in the combatant countries. https://en.wikipedia.org/wiki/Roger_Bannister https://en.wikipedia.org/wiki/Roger_Bannister