1940s vs decade

1940s and decade both are nouns.

Word	Noun	Adjective	Verb	Adverb
1940s	Yes	No	No	No
decade	Yes	No	No	No

As nouns, decade is a hypernym of 1940s; that is, decade is a word with a broader meaning than 1940s:

Other hypernyms of 1940s include decennary, decennium.

1940s (noun)	decade (noun)
the decade from 1940 to 1949	the cardinal number that is the sum of nine and one; the base of the decimal system
	a period of 10 years