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The story was conceived, however, by the Pixar brain trust: Lasseter, Finding Nemo‘s Andrew Stanton, Up and Monsters Inc.‘s Pete Docter, and Toy Story 3‘s Lee Unkrich
Fans reacted to the news with what can only be described as uncertainty, taking to Twitter to wring their hands in worry. Most people, including many within Pixar, felt Toy Story 3 was a perfect close to a beloved trilogy, and are hesitant about anything that might sully that legacy.
In a statement, Lasseter tried to calm those nerves: “We love these characters so much, they are like family to us,” he wrote. “We don’t want to do anything with them unless it lives up to or surpasses what’s gone before.”
He acknowledged that Toy Story 3 “ended Woody and Buzz’s story with Andy so perfectly that for a long time, we never even talked about doing another Toy Story movie. But when Andrew, Pete, Lee and I came up with this new idea, I just could not stop thinking about it. It was so exciting to me, I knew we had to make this movie — and I wanted to direct it myself.”
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We'll see about that.
http://insidemovies.ew.com/2014/11/06/john-lasseter-will-direct-toy-story-4-breaking/