The very first move the Yankees made last offseason was re-signing Chris Young to be their fourth outfielder. They did that days after the end of the World Series. This offseason their first move was acquiring Aaron Hicks to replace Young. Probably a coincidence, or maybe the Yankees just really value quality fourth outfielders.
Brett Gardner trade rumors were flying at the time of the Hicks trade, so for a while it seemed he could end up in a starting role. Gardner is still with the Yankees, so Hicks remains the fourth outfielder for now, though it’s starting to become clear the team intends to play him a fair amount.