ETYM Old Eng. fourthe, ferthe, feorthe, AS. feórtha, from feówer four.
(Homonym: forth).
Coming next after the third and just before the fifth in position or time or degree or magnitude; SYN. 4th, quaternary.

In music, an interval of four diatonic notes consisting of two whole tones and a semitone, for example C–F. A fourth is an example of a “perfect interval” because it remains the same in both major and minor keys. A fourth may be augmented (increased by a semitone), for example C–F sharp.

1. Following the third position; number four in a countable series.
2. The musical interval between one note and another four notes away from it.