Shikoku vs island

Shikoku and island both are nouns.

Word	Noun	Adjective	Verb	Adverb
Shikoku	Yes	No	No	No
island	Yes	No	No	No

Shikoku (noun)	island (noun)
the smallest of the four main islands of Japan; to the south of Honshu and to the east of Kyushu; separated from Honshu by the Inland Sea; forested and mountainous	a zone or area resembling an island
	a land mass (smaller than a continent) that is surrounded by water