senior vs fourth-year

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

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

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

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

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

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