fourthly vs fourth

fourthly is not a noun while fourth is a noun.

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

As adverbs, fourthly and fourth are synonyms defined as:

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

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

fourthly (adverb)	fourth (adverb)
in the fourth place	in the fourth place