petit four vs cake

petit four and cake both are nouns.

Word	Noun	Adjective	Verb	Adverb
petit four	Yes	No	No	No
cake	Yes	No	Yes	No

As nouns, cake is a hypernym of petit four; that is, cake is a word with a broader meaning than petit four:

petit four: small (individual) frosted and ornamented cake
cake: baked goods made from or based on a mixture of flour, sugar, eggs, and fat

petit four (noun)	cake (noun)
small (individual) frosted and ornamented cake	a block of solid substance (such as soap or wax)
	baked goods made from or based on a mixture of flour, sugar, eggs, and fat
	small flat mass of chopped or ground food

petit four (verb)	cake (verb)
	form a coat over