fourth vs fourthly

fourth is a noun but fourthly is not a noun.

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

As adverbs, fourth and fourthly are synonyms defined as:

fourth (noun)	fourthly (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

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

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