plus fours vs knee breeches

plus fours and knee breeches both are nouns.

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

As nouns, knee breeches is a hypernym of plus fours; that is, knee breeches 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 breeches: (used in the plural) trousers ending above the knee

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

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