I think in my mind, Four looked like a mix of Josh Duhamel and Liam Hensworth. Well, that's what I've been imagining lately. The pictures I've seen of Theo James, I think he'll fit this not perfectly, but good enough.
Theo James is a 28 year old English actor with such credits as Underworld: Awakening, Bedlam, Downton Abbey, and A Passionate Woman. I've never actually seen him act so I can't comment on how he'll do. I'm not sure how he fits with Shailene Woodley, but then, I think Four isn't supposed to be this nice perfect fit with Tris.
According to other articles across the 'net Kate Winslet, Aaron Eckhart, Ray Stevenson, and Miles Teller have been signed to roles to which have not been disclosed as of yet. According to hypable.com, Winslet's role is as Jeannie Matthews, but this is the only site that names the role. IMDB.com also has her listed as Matthews, but with no source I can only assume they got it from hypable.com.
Kate Winslet is a 37 year old British actress with such credits as Titanic, Sense and Sensibility, Hamlet, Eternal Sunshine of the Spotless Mind, Finding Neverland, and All the King's Men.
Aaron Eckhart is a 45 year old American actor who moved to England at the age of 13. He has been in such movies as Any Given Sunday, Erin Brockovich, Paycheck, and Battle: Los Angeles.
Ray Stevenson is a 48 year old Irish actor with such credits as Band of Gold, The Book of Eli, Thor, and GI Joe: Retaliation.
Miles Teller is a 26 year old American actor with such credits as Footloose, Project X, and 21 & Over.