Woody, Buzz and Jessie are returning nine years after they said goodbye to Andy and settled into their new home with Bonnie at the end of Toy Story 3.
The Hollywood Reporter's Todd McCarthy said: "It's now certain what one of the summer's blockbusters will be.
"More than that, how many other film series can legitimately claim to have hit four home runs in a row?" he added.
Variety's Peter Debruge said the movie gives "satisfying emotional closure", adding that "the fourth movie wraps up the saga beautifully".
He added the film "explores the idea of purgatory: What's it like for a plaything to be ignored, overlooked or entirely unused?"
Toy Story 4, which is released in the UK on 21 June, sees the toys grapple with the idea of the jumble sale crate, and the possibility of being left without a purpose when their owners grow up.
It also introduces Forky, a spork who has been given a makeover in a craft project and is in denial about the fact he's now a toy.
However The Guardian's Peter Bradshaw awarded it just three stars, adding the film is "a repeat of earlier ideas and plotlines".