What group of European countries make up the Visegrad Four?

The Visegrad Group, also known as the Visegrad four, or simply V4, is a political group of neighboring countries. The group was originally 3 countries, but after the breakup of Czechoslovakia into the Czech Republic, and Slovakia it became four. The group formed in February of 1991, when Hungary, Poland, and Czechoslovakia met in Visegrad Hungary. Visegrad Hungary, is the home of an ancient Hungarian castle, that overlooks the Danube river, and controlled the area. The group's purpose was to promote their integration into the European Union. All 4 members; Hungary, Poland, Slovakia, and Czech Republic, did join the EU in May of 2004.