fourth meaning
EN
[fɔːθ] [fɔɹθ] [fo(ː)ɹθ] [foəθ] [-ɔː(ɹ)θ] 
US WFourth
- Fourth or 4th can refer to:
- Fourth, the ordinal number following third
- fraction (mathematics), one quarter or ¼, equal to 0.25
- Preceded by the, refers to the United States holiday of Independence Day, celebrated on the Fourth of July.
EN Fourth 
- NounPLfourths
- Adjective
- The ordinal form of the number four.
- The ordinal form of the number four.
- More Examples
- Used in the Middle of Sentence
- ... the simplest thing John Prescott could do, to help secure a fourth term for Labour, would be to disapparate, taking his fellow revenants with him. ( - The Observer)
- Are you listening, sir? Or did your mind drift / to the next country in your axis, / to all the cool bomb drops you get to call: / delivery fresh from the Fourth Reich to y’all?
- The second Book is divided into eight Chapters, treats of the origin of the Greek Characters, and changes that happen'd in them, to the fourth Century of the common Era.
- Used in the Ending of Sentence
- The false primary about one third the length of the first, which is shorter than the fourth.
- Used in the Middle of Sentence
Definition of fourth in English Dictionary
- Part-of-Speech Hierarchy
- Adjectives
- Uncomparable adjectives
- Uncomparable adjectives
- Nouns
- Countable nouns
- Countable nouns
- Adjectives
- en fourths
- en fourthly
- en fourthness
- en fourth-wall
- en fourth gear
Source: Wiktionary
Grammatically, this word "fourth" is an adjective, more specifically, an uncomparable adjective. It's also a noun, more specifically, a countable noun.