fourth / fôr[unvoicedth]/ • ordinal number constituting number four in a sequence; 4th: the fourth and fifth centuries | there were three bedrooms, with potential for a fourth. ∎  (a fourth/one fourth) a quarter: nearly three fourths of that money is now gone. ∎  the fourth finisher or position in a race or competition: he could do no better than finish fourth. ∎  the fourth (and often highest) in a sequence of a vehicle's gears: he took the corner at the end of the road in fourth. ∎  the fourth grade of a school. ∎  fourthly (used to introduce a fourth point or reason): third, visit popular attractions during lunch; fourth, stay late. ∎  Mus. an interval spanning four consecutive notes in a diatonic scale, in particular (also perfect fourth) an interval of two tones and a semitone (e.g., C to F). ∎  Mus. the note that is higher by this interval than the tonic of a diatonic scale or root of a chord.

fourth 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.

fourth. Interval in melody or harmony in which there are 4 steps in the major or minor scale from one extreme to the other (incl. the bottom and top notes). Perfect 4th is the distance from C up to F or from G to C; a semitone more gives the augmented 4th (e.g. C up to F♯).