fourth stomach vs tummy

fourth stomach and tummy both are nouns.

Word	Noun	Adjective	Verb	Adverb
fourth stomach	Yes	No	No	No
tummy	Yes	No	No	No

As nouns, tummy is a hypernym of fourth stomach; that is, tummy is a word with a broader meaning than fourth stomach:

fourth stomach: the fourth compartment of the stomach of a ruminant; the one where digestion takes place
tummy: an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion

Other hypernyms of fourth stomach include breadbasket, stomach, tum.

fourth stomach (noun)	tummy (noun)
the fourth compartment of the stomach of a ruminant; the one where digestion takes place	an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion
	slang for a paunch