The film is due to be released next year, eight years after the previous movie.
Tim Allen, who plays the "To infinity and beyond" favourite, did not go into detail but said the Matrix star had a "great part", before gesturing with his fingers to indicate a small toy.
A trailer released earlier this month confirmed Tom Hanks would be returning as Sheriff Woody and Joan Cusack as Jessie.
Appearing on The Tonight Show, Allen, who first played Buzz in the 1995 original, said recording the final scene of the upcoming film was very emotional.
He said: "Hanks said it best - of course, he always says stuff best - but the last scene of the movie I couldn't even get through it. I thought there were pages after it.
"I read it and I had to turn around from reading it in the studio, it was so emotional."
Referring to the famous final scene of the movie classic Gone With the Wind, Allen added: "It's a 'Scarlett, I don't give a damn moment'."
Along with Buzz and Woody, other Toy Story stalwarts including Mr Potato Head, Rex, Hamm and Slinky Dog return in the latest instalment. A new character, a spork with googly eyes called Forky, is also part of the line-up.
Allen, 65, known for his role in US sitcom Home Improvement, said the film would surprise fans.
He said: "It is so good, these guys are geniuses, it's so deep and moving and a wonderful plot. I can't give anything away, but it's about different kinds of toys.
"They will really surprise you with the toys they come up with, characters come back, new guys (are) in it."
Toy Story was released in 1995 and was the first ever fully computer animated feature film. It was nominated for three Oscars.
Toy Story 4 will be released in cinemas on 21 June next year.