fourth stomach vs abomasum

fourth stomach and abomasum both are nouns.

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

As nouns, fourth stomach and abomasum are synonyms defined as:

fourth stomach and abomasum: the fourth compartment of the stomach of a ruminant; the one where digestion takes place

fourth stomach (noun)	abomasum (noun)
the fourth compartment of the stomach of a ruminant; the one where digestion takes place	the fourth compartment of the stomach of a ruminant; the one where digestion takes place