From BR Bullpen
A AAAA player, or four A player, but rarely quadruple A, is a player who is an outstanding player at the AAA level but always has trouble succeeding at the major league level, either because of a lack of one of the five major tools or bad luck.
Many followers of sabermetrics question the idea that there are players who can dominate the high minors but fail in MLB. They think that teams tend to give up on such players too easily, leaving a population of potentially valuable players languishing in the minors. Bill James described such players as Ken Phelps All-Stars after the prototypical Ken Phelps, a AAAA player who finally made good when given a chance a number of years after he had first dominated competition at the AAA level.