It was always going to be hard to top Season 3, with it's fantastic climax of Nucky under threat, and they didn't manage it. Best thing about this was the addition of Jeffrey Wright as a kind of black power, highly educated gangster - a different sort of black gangster to what we've seen before. His clashes with Chalky White, Nucky and so on were very exciting. Stephen Graham's Al Capone is always interesting.
Far less gripping is the psychotic FBI agent Tolliver, and stuff involving Nucky's nephew, and the Gretchen Moll stuff with Ron Livingstone (though this does have a decent pay off) and Nucky's adventures in Florida with Patricia Arquette. They ran out of stuff for Kelly MacDonald to do. Michael Shannon always looks as though he's going to do something interesting but never quite crashes through - I expected him to be running his own gang by now. And you get the feeling when they ran out of ideas they just decided to kill someone off - like Harrow or Eddie. Always watchable, just a dip in quality - I think they'd run out of things to say.