The actor voices Buzz Lightyear in the Disney and Pixar movies, the fourth of which is in the works.
Toy Story 3 famously left many fans in tears – and it seems the next instalment will be no different.
He told The Talk when asked to discuss it: “Yes, I gotta resist getting emotional.
“I don’t want to give it away, but this is an incredibly great story.”
He added: “It is so emotional, it’s so funny, it’s so big, the idea they’ve come up with, I’m startled.
“I couldn’t even get through the last scene. I would love to be a Washington leaker.”
Asked if he could give anything more away, he insisted: “I just can’t do it. I can’t give any more away.
“They’ve got great characters but a couple of scenes toward the end were really hard to get through.”
Back in the cast once again are Tom Hanks as Woody, Joan Cusack as Jessie – and, in what is thought to be a key role this time around, Annie Potts as Bo Peep.
Michael Keaton, Bonnie Hunt, Timothy Dalton and Laurie Metcalfe are also on the cast list.
Don Rickles was set to reprise the role of Mr Potato Head but, at the time of his death in 2017, his representative confirmed that he had not recorded any new lines.
The animation is due in the summer of 2019, after its release was switched with this summer’s smash hit Incredibles 2.
Allen’s words will be reassuring to those who heard earlier this year that “three quarters” of the script had been completely scrapped and rewritten.
Rashida Jones and Will McCormack were originally hired to write it, but withdrew in late 2017 due to “philosophical differences”. Stephanie Folsom took over.
The Toy Story films are among the most successful of all time: Toy Story 3 took a jaw-dropping $ 1.1 billion at the box office in 2010.
Toy Story 4 is out on June 21, 2019.