Injuries will happen but I think the claim that they average 450-500 PA overstating it just a little bit.
From a Baseball Prospectus article:
From an old write up on Tyrone Taylor from Baseball Reference:
In addition, a hitter needs 502 PA in 162 games to be a qualified regular. It seems implausible that the average 4th outfielder is a qualified regular.
However I do agree that there is a significant chance that one or more of your starting 3 OF gets injured, but not all of those PA will go to the opening day 4th OF - they will be spread around him, the injury call up, prospects, etc.