Fourth dimension a postulated spatial dimension additional to those determining length, area, and volume; the phrase is recorded from the late 19th century, and is now also used in physics to denote time as analogous to linear dimensions.
fourth estate the press; a group regarded as having power in the land equivalent to that of one of the three Estates of the Realm, the Lords spirtual and temporal and the House of Commons. The term derives from a usage by Thomas Babington Macaulay (1800–59), ‘The gallery in which the reporters sit has become a fourth estate of the realm.’
Fourth of July in the US, Independence Day.
Fourth of June the birthday of George III (1738–1820), speech day at Eton because of his interest in the school.
Fourth World those countries and communities considered to be the poorest and most underdeveloped of the Third World.
see also Close Encounter of the Fourth Kind at close.