four vs digit

four and digit both are nouns.

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

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

Other hypernyms of four include figure.

four (noun)	digit (noun)
a playing card or domino or die whose upward face shows four pips	a finger or toe in human beings or corresponding body part in other vertebrates
the cardinal number that is the sum of three and one	one of the elements that collectively form a system of numeration
	the length of breadth of a finger used as a linear measure

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