From various sources, it has been mentioned, by Gale Ann Hurd, Norman Reedus, and Greg Nicotero, that the walkers will make a more prominent appearance in Season 4 and also become more dangerous because of an unknown enhancement made to them.
This is basically what I'm asking, what do you think this addition is? I think it's very interesting and mysterious - and all I can think of is that they've figured how to wield machine guns or something, of course that's very absurd, and makes them like humans. You guys probably have better ideas! What do you think?