Fourth-person Definition

noun

(grammar) A variety of the third person sometimes used for indefinite referents, such as one in one shouldn't do that.

Wiktionary

(linguistics) Grammatical person in some languages distinct from first, second, and third persons, semantically translated by one of them in English. For example, in Ojibwe, it is a variety of the third person used for a secondary, or obviative, third person.

The man saw the dog as he was leaving. (if he is the third person, or proximate, it refers to the man; if he is the fourth person, or obviative, it refers to the dog)

Wiktionary