What's the difference between the iPad 3 (retina) and this 4th generation they're talking about? Are they referring to an iPad Mini as 4th gen or are they saying the retina iPad (3rd gen) is 4th gen now?
In any case: The third-generation iPad ("The new iPad") was released last Spring. The fourth-generation iPad ("iPad with Retina Display") was released last Autumn. The iPad Minis internally have the same major version number at the iPad 2s. There isn't much difference between the third and fourth generations, other than a new port and a slight spec bump.