Now this isn't a knock on the guy. I think he's a borderline HOF'er. But the talk of the "Core Four" with Posada's stature in the five Yankee championships isn't quite as high as people like to think.
In the first ring in 1996, Posada wasn't part of the core. Joe Girardi was the starting catcher. Jim Leyritz was the backup catcher. Posada only played 4 games behind the plate that year, appeared in 8 games and had 14 at bats with only 1 hit.
In 1997, when the Yankees were knocked out in the first round, Posada was the backup.
In 1998, when the Yankees beat the Padres in the World Series, in his first season as the most used catcher, playing in 111 games, 99 of them behind the plate. Joe Girardi played 78 games behind the dish. In the postseason, he had 22 at-bats, compared to Girardi's 21.
He was the starting catcher for the 1999 and 2000 World Series winning teams.
None of this is no slouch and isn't discounting his career by any stretch of the imagination. But he really can't be considered a 5 ring guy. He's a 4 ringer - 1998, 1999, 2000 & 2009.