According to The Wrap, actor Tim Allen is signed for a fourth Toy Story film. Whether or not Disney/Pixar will move on a new movie is another story entirely.
The trade states that many of the films voice actors could possibly have another film attached to their respective contracts, but that doesn't necessarily mean that the studio is obligated to put another feature-length sequel into production. We already know that Woody, Buzz, Mr. Potato Head and the rest of the toy gang will appear in a Pixar short that will play in front of the studio's Cars 2 next summer and that the global popularity of the franchise justifies further application of the brand, but according to Toy Story 3 director Lee Unkrich, the series feels like it is complete.
Says Unkrich: "I really tried my best to end the story of Andy and his toys and bring that story to a close in a really nice way at the end of ['Toy Story 3']. That being said, we know that people love the characters, love Woody and Buzz, and would hate to say good-bye to them completely."
I know that the Toy Story characters will live in my heart forever and many fans will agree. The third chapter is currently the highest grossing domestic release of the year and that certainly should give Disney brass a reason to consider making another film, but if they are going to do it, I'd suggest pulling the trigger on this one quicker. It was 11 years in between TS2 and TS3, and I don't think that anyone wants to wait until 2020 for another animated adventure.
Source: The Wrap