The movie will be helmed by "Limitless" and "The Illusionist" director Neil Burger from a script by Vanessa Taylor.
It is to begin principal photography next month in Chicago and is set for a 2014 release.
"Theo is not only an incredibly talented actor, he is also who we envisioned as Four when reading Veronica's novel which has taken the world by storm. As we continue to develop the film, the studio remains committed to providing fans with a movie adaptation that stays as true to the book as possible and we are confident that we have done so with our selection of Shailene and Theo in the leading roles," Rob Friedman and Patrick Wachsberger, co-chairmen of Lionsgate Motion Picture Group, said in a joint statement Friday.
"We took our time to find the right actor to fill the role of Four, and Theo is definitely the perfect fit," said Erik Feig, Lionsgate Motion Picture Group's president of production. "Veronica has crafted a truly iconic character in Four and we cannot wait to begin production and bring him and this story to life for millions of fans around the world."