It doesn't matter whether it is Toy Story 2 or Toy Story 15, as long as it is a good story. Two and Three worked because they were good. That is all I ask. They need to do new stories at the same time or things will get stale.
At first my reaction was "No, the 3rd ended the whole series perfectly! Just stay with the Shorts and Specials." I've seen the people involved so when I go to see it, I will have an open mind, but I just don't want the 4th installment to damage an already perfect trilogy.
But I came across this article that sheds a little light on the 4th movie: