grammar

"Ago" as it is used at present can function as adverb, adjective or preposition.

As adverb: Long ago (modifying adjective)

As adjective : Years ago (qualifying noun)

As preposition : Dinosaurs died out 65 million years ago.(linking a noun phrase)

Though born out of "agone", in its present day usage, 'ago' is more an adverb or preposition than an adjective (in a restricted sense). 'Ago' refers to completed event in the past. Preposition — though the literal meaning may be misleading —are words that precede(mostly)or follow (they are just a handful!) a noun phrase and form adverbials with them. Some prepositions like 'before', 'about' or 'around' can be placed in either position. An example of postposition is AGO

In the phrase FOUR SCORE AND SEVEN YEARS AGO, 'ago' is a preposition linking the noun phrase 'four score and seven years' to the rest of the sentence. Had it been an adverb, it would not serve this linking function.

So the phrase is prepositional in construction irrespective of the position of " AGO" relative to the compliment,'four ... Years' , and adverbial in function.