plus fours vs knee pants

plus fours and knee pants both are nouns.

Word	Noun	Adjective	Verb	Adverb
plus fours	Yes	No	No	No
knee pants	Yes	No	No	No

As nouns, knee pants is a hypernym of plus fours; that is, knee pants is a word with a broader meaning than plus fours:

plus fours: men's baggy knickers hanging below the knees; formerly worn for sports (especially golf)
knee pants: (used in the plural) trousers ending above the knee

Other hypernyms of plus fours include breeches, knee breeches, knickerbockers, knickers.

plus fours (noun)	knee pants (noun)
men's baggy knickers hanging below the knees; formerly worn for sports (especially golf)	(used in the plural) trousers ending above the knee