I don't know why they didn't stick with just iPad 3 and iPad 4. Maybe people have different reactions to version numbers?

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.