Three of the four players were Diamondbacks. The fourth was chosen because the Diamondbacks contacted his team to ask how he accomplished his feat of defending the outfield. After they defended the outfield (albeit they split their time between outfield and infield), each of the four players returned to an almost exclusive focus on infield defense.
After looking at each of the four players, we will look at results they have in common during and after the seasons when they defended the outfield.
Knowing something often requires measuring something. As we are looking at defense, the chosen measure of effectiveness is Defensive Runs Saved (DRS) per The Fielding Bible.