fourth-year vs senior

fourth-year is not a noun while senior is a noun.

Word	Noun	Adjective	Verb	Adverb
fourth-year	No	Yes	No	No
senior	Yes	Yes	No	No

As adjectives, fourth-year and senior are synonyms defined as:

fourth-year and senior: used of the fourth and final year in United States high school or college

fourth-year (noun)	senior (noun)
	an undergraduate student during the year preceding graduation
	a person who is older than you are

fourth-year (adjective)	senior (adjective)
used of the fourth and final year in United States high school or college	older; higher in rank; longer in length of tenure or service
	used of the fourth and final year in United States high school or college
	advanced in years; (`aged' is pronounced as two syllables)