4 vs digit

4 and digit both are nouns.

Word	Noun	Adjective	Verb	Adverb
4	Yes	Yes	No	No
digit	Yes	No	No	No

As nouns, digit is a hypernym of 4; that is, digit is a word with a broader meaning than 4:

Other hypernyms of 4 include figure.

4 (noun)	digit (noun)
the cardinal number that is the sum of three and one	a finger or toe in human beings or corresponding body part in other vertebrates
	one of the elements that collectively form a system of numeration
	the length of breadth of a finger used as a linear measure

4 (adjective)	digit (adjective)
being one more than three