Some plot points in Toy Story 4 are discussed below, but nothing you can't really glean from the trailers.
Toy Story 4 raises an odd metaphysical question: Are we what we are solely because of another's designation?
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Consider the strange, twisted tale of Forky (Tony Hale). A spork rescued from the trash and combined with some googly eyes, a pipe cleaner, popsicle stick, and clay, Forky, aside from being incorrectly named (shouldn't it be Sporky?) by his creator, gains sentience upon having "Bonnie" scrawled upon his deformed feet.
For Forky, this is a horrifying fact. But it should be mildly disturbing for the rest of us as well. After all, if that's all it takes for these inanimate objects to gain sentience, are there not a whole discarded class of objects that earn self-awareness after brief bouts of servitude as playthings? Does every stick that gets picked up and used as a Stormtrooper's blaster spend the rest of its existence questioning its horrifying existence as a piece of rotting wood destined for the chipper? If a small child picks up a piece of poop and treats it like a toy, does it turn into Shitty and yearn longingly for the compost heap?
Odder still, Forky seems to have some memory of his pre-life: He wants to live in trash, where it's warm and safe. He is trash. That's his role. To be used once and discarded.
And this is what Toy Story 4 is about, more or less: knowing one's role in the world, one's place and what to do with oneself, one's responsibilities, and to whom one owes those responsibilities. For Forky, that abomination before the eyes of God, this means learning that he has responsibilities as a toy, to provide comfort and care to the little girl who brought him to life. For Woody (Tom Hanks), it means something different: knowing when the time for care has passed and when one must let go.
All of which is to say that Toy Story 4 is really about parenthood. Woody waxes poetic about having watched one of "his" kids grow up—Andy, the owner from the first three movies—and feeling less useful the second time around, with Bonnie. She cares for other toys. Woody's tired. He doesn't have much left to do; other than convincing Forky of his purpose and making sure he doesn't run off into the garbage, leaving Bonnie bereft, Woody is adrift. One kid has grown; the other grown distant from him.
Is he still Bonnie's toy, really, if she has no use for him—if his designation has changed?