4WD vs transmission system

4WD and transmission system both are nouns.

Word	Noun	Adjective	Verb	Adverb
4WD	Yes	No	No	No
transmission system	Yes	No	No	No

As nouns, transmission system is a hypernym of 4WD; that is, transmission system is a word with a broader meaning than 4WD:

4WD: a transmission that provides power directly to all four wheels of a motor vehicle
transmission system: the gears that transmit power from an automobile engine via the driveshaft to the live axle

Other hypernyms of 4WD include transmission.

4WD (noun)	transmission system (noun)
a transmission that provides power directly to all four wheels of a motor vehicle	the gears that transmit power from an automobile engine via the driveshaft to the live axle
a motor vehicle with a four-wheel drive transmission system