At the beginning, when Andy gives Woody to Bonnie, she reacts differently from when the same event was depicted in Toy Story 3 (2010).
At the beginning, while Woody is in the closet, the letters on his boot disappear.
When Woody is talking to Forky on the road, Forky's arms move up and down his body between shots, by his head, then halfway down his body, then back by his head.
When Bonnie started to fall asleep, she was wearing her clothes. But when she woke up in the morning, she was wearing her pajamas.
When Bonnie fell asleep, she was sitting at the table. But when she woke up in the morning, she was in the bed.
At the very beginning of the film, the calendar on Andy's bedroom wall shows September having 31 days.
While there were indeed talking dolls with a voice box that is easy to remove if not user-serviceable, given Gabby's age as presumably an early-model Gabby Gabby doll, it would be in reality extremely difficult to remove and reinstall her voice box. Not to mention excruciating for both Gabby and Woody.
Bo Peep's timeline is full of mistakes. She was donated nine years before the film. When she meets Woody she says she's been "lost" for seven years. Based on various comments, in that two-year time span she was given to a little girl who grew up to not need her and spent years in the antiques store before getting bored enough to go on her own. Bo is part of a bedside lamp. It is easily conceivable that she was bought for a little girl who was afraid of the dark but had outgrown the fear two years later. The seven years after that include her time in the antiques store, because she says that she has been out on her own (i.e. without a kid) for seven years, not that she has been living the life she does now (i.e. without waiting for or even wanting a kid) for seven years.
At the beginning, Forky doesn't know he is a toy, nor desires to be one, but he follows the toy rule of acting inanimate when humans are looking at him. However, as he is one, it may be natural for him without him even knowing or desiring.
When the gang is trying to escape Gabby Gabby's cabinet, Woody is held by his extended voicebox string. Yet one of his voicebox lines plays. The string has to be taken back up into the voicebox in order for the record to play.
Every toy seems to be alive except the red toy truck Woody uses at 34:47 to slide down the road during his escape from the playground.
At the end of Toy Story 3 (2010), Andy's house only has one driveway. During the flash back in the beginning of Toy Story 4, there are two driveways - one leading up to the garage and now one on the left side of the house.
It's unknown if Bonnie found out that Woody was gone.
When Bonnie and her mom left the antique store, they forgot Bonnie's backpack. And when they returned to the RV, they didn't notice that the backpack was missing.
Near the beginning of the movie, it's revealed that Bonnie was playing with Woody less and less. But when Bonnie and her parents went on a road trip, Woody was one of the toys that she brought. She also played with him when they went to Poultry Palace.
Just like in the 3rd movie, Woody and Buzz's voice boxes were working properly, despite the fact that it had been 12 years since the events of Toy Story 2.
Even though Woody told Bo Peep that he was having a great time with Bonnie, he didn't tell her that she left him in the closet, and that she didn't play with him anymore.
Bonnie's dad is guilty of dangerous driving. Unaware that living toys are affecting the RV, to him it's just responding erratically to the controls. Despite that dangerous situation, he attempts to drive forward instead of stopping the vehicle in neutral with his foot hard down on the brake pedal, and the ignition switched off.