gun vs four-pounder

gun and four-pounder both are nouns.

Word	Noun	Adjective	Verb	Adverb
gun	Yes	No	Yes	No
four-pounder	Yes	No	No	No

As nouns, four-pounder is a hyponym of gun; that is, four-pounder is a word with a more specific, narrower meaning than gun:

Other hyponyms of gun include cannon, field artillery, field gun, depth charge thrower.

gun (noun)	four-pounder (noun)
the discharge of a firearm as signal or as a salute in military ceremonies	an artillery gun that throws a shot weighing four pounds
a weapon that discharges a missile at high velocity (especially from a metal tube or barrel)
large but transportable armament
a hand-operated pump that resembles a pistol; forces grease into parts of a machine
a pedal that controls the throttle valve
a professional killer who uses a gun
a person who shoots a gun (as regards their ability)

gun (verb)	four-pounder (verb)
shoot with a gun