tetragram vs written word

tetragram and written word both are nouns.

Word	Noun	Adjective	Verb	Adverb
tetragram	Yes	No	No	No
written word	Yes	No	No	No

As nouns, written word is a hypernym of tetragram; that is, written word is a word with a broader meaning than tetragram:

tetragram: a word that is written with four letters in an alphabetic writing system
written word: the written form of a word

tetragram (noun)	written word (noun)
a word that is written with four letters in an alphabetic writing system	the written form of a word