four-pounder vs heavy weapon

four-pounder and heavy weapon both are nouns.

Word	Noun	Adjective	Verb	Adverb
four-pounder	Yes	No	No	No
heavy weapon	Yes	No	No	No

As nouns, heavy weapon is a hypernym of four-pounder; that is, heavy weapon is a word with a broader meaning than four-pounder:

Other hypernyms of four-pounder include artillery, gun, ordnance.

four-pounder (noun)	heavy weapon (noun)
an artillery gun that throws a shot weighing four pounds	large but transportable armament