Baker wouldn't be LEADING S-M:4, it will be whoever's playing Parker/S-M. True, Baker is not a leading man, in Drive, he was a dying father who wanted to go on the cross-country race with his daughter, his last hurrah, as it were. Other than Drive, I haven't seen Baker in anything.
Baker had a lead role in Happiness a while back http://www.imdb.com/title/tt0147612/ and I thought he was fantastic. If he's given the right script he can really give a tremendous performance.