four vs quadruplet

four and quadruplet both are nouns.

Word	Noun	Adjective	Verb	Adverb
four	Yes	Yes	No	No
quadruplet	Yes	No	No	No

As nouns, four and quadruplet are synonyms defined as:

four and quadruplet: the cardinal number that is the sum of three and one

Other synonyms of four include 4, foursome, IV, Little Joe, quartet, quatern, quaternary, quaternion, quaternity, tetrad.

four (noun)	quadruplet (noun)
a playing card or domino or die whose upward face shows four pips	a set of four similar things considered as a unit
the cardinal number that is the sum of three and one	one of four children born at the same time from the same pregnancy
	the cardinal number that is the sum of three and one

four (adjective)	quadruplet (adjective)
being one more than three