It doesn't matter what they do with the story. They created a movie that they want to give them money. It is a shameless cash grab, and they know that. There was no real reason to make a movie after the third one. The plot is secondary to what they want from it. There's no real justification to it. But, if you want my guess, I'd say that Bonnie lost some of the toys somehow, and Woody has to help them get home or something.