In the book, readers rejoin Bridget in her 50s, a mother to two children and widowed following the death of her husband Mark Darcy.
Darcy was played by Colin Firth in the earlier Bridget Jones movies.
The production studio has not commented on how closely the new film will follow the storyline of its paperback namesake, but it has been revealed author Helen Fielding has written the script.
Speaking on the Graham Norton show, Grant indicated that Fielding had used a real-life tragedy to come up with the plotline.
“She had a sad story,” he said.
“She got married to an American screenwriter, she had children and then he died.”
Grant said that Fielding then started writing a novel about a woman raising children by herself, and then realised the main character was “a bit like Bridget”.
“So she made it into a Bridget Jones book,” he said.
“And as well as being extremely funny, it’s very, very sad.”
Grant, who is best known for starring in romcoms including Notting Hill and Four Weddings and a Funeral, did not appear in the third film.
He said that initially, he felt there was “absolutely no role” for his character, Daniel, in the fourth version either.
“But they wanted to cram me in,” he said.
So he said he sat down with producers to help shape the character, who was known in the earlier films for being a serial womaniser.
“I felt that what they proposed was fine, but not great.
“And I felt that he needed a third dimension, he’s in his 60s now, you can’t just have him smoothing his way down King’s Road eyeing up young girls.
“Something needs to have happened to him in the interim.
“So we invented a rather good – I invented a rather good – interim story.”
The final version got his approval, he said.
“It’s actually a very good and moving script. And I say that as someone who’s horrid about scripts. This was brilliant.”
