Rumors have it said that both the iPad 3 and iPad 4 will be launched in the same time sometime this year, with 2 versions that is for the high end users (iPad 4) and for the mid-to-low-entry users (iPad 3).
However this is only a rumor and whether both iPad 3 and iPad 4 will launch at the same time, or only the iPad 3 will launch, is solely dependent on Apple marketing strategy. So we will see how Apple will announce its upcoming iPad later this year.