adjective

1. preceded by three others in a series; 4th
2. designating any of the four equal parts of something
3. designating the forward gear ratio of a motor vehicle next after third gear

Etymology: ME feorthe, ferthe < OE feortha: see four & -th

noun

1. the one following the third
2. any of the four equal parts of something;
3. Music
   1. the fourth tone of an ascending diatonic scale, or a tone three degrees above or below any given tone in such a scale; subdominant
   2. the interval between two such tones, or a combination of them

adverb