fourth vs fourth part

fourth and fourth part both are nouns.

Word	Noun	Adjective	Verb	Adverb
fourth	Yes	Yes	No	Yes
fourth part	Yes	No	No	No

As nouns, fourth and fourth part are synonyms defined as:

Other synonyms of fourth include one-fourth, one-quarter, quarter, quartern, twenty-five percent.

fourth (noun)	fourth part (noun)
the musical interval between one note and another four notes away from it	one of four equal parts
one of four equal parts
following the third position; number four in a countable series

fourth (adjective)	fourth part (adjective)
coming next after the third and just before the fifth in position or time or degree or magnitude

fourth (adverb)	fourth part (adverb)
in the fourth place