The Schengen area (which has common visa rules) is not the same as the EU. Those four countries are part of the Schengen agreement even though they are not in the EU. Conversely, Ireland is not included because although it is an EU member, it is not in the Schengen zone.

The EU is a very complicated thing.. You're looking for the Schengen countries (inside the yellow line). Romania. Bulgaria, Croatia and Cyprus are the four non-Schengen countries that have also opted in.