4. Preposition
"Forth" is a preposition, that is to say, a function word that usually expresses a relation to another word or phrase in a sentence, they may show direction, location, time, introduce an object, etc.
In this excerpt, the "forth" word is acting as the proposition within the phrasal verbs "Brought forth" which means to cause something to happen or be seen or known.
For further clarification, a phrasal verb consists of a verb and a preposition or an adverb (or both), that when are together express a different meaning than the meaning of words being separated.