plus fours vs plural

plus fours and plural both are nouns.

Word	Noun	Adjective	Verb	Adverb
plus fours	Yes	No	No	No
plural	Yes	Yes	No	No

plus fours (noun)	plural (noun)
men's baggy knickers hanging below the knees; formerly worn for sports (especially golf)	the form of a word that is used to denote more than one

plus fours (adjective)	plural (adjective)
	grammatical number category referring to two or more items or units
	composed of more than one member, set, or kind