Ttyjfyk

I was reading a Wikipedia article about the Slavey (Slave) language in Canada, and it says that Slavey has first, second, third and fourth person. I've never heard about a language having a fourth person, so I was just wondering if someone here knows when is this used and how it works? Or, since it isn't very likely you're familiar with this particular language, just in general, what does a fourth person in a language denote?

The fourth person is a (rare) synonym for the obviative. In languages with this feature, when there are two third-person referents and one of them is less salient, the less salient one may be marked as obviative and the more salient one as proximative. According to Rice (1989), the fourth-person pronoun go- is used for objects when the subject is third person (sorry, the source doesn't provide morpheme boundaries or a morpheme-by-morpheme gloss, and I'm not familiar with American languages):

(1) nágoneht'u
 'S/he is hitting them(human).'

The fourth-person pronoun ye- is used for third-person direct nonhuman objects when the subject is third person, as a fourth-person possessor (Rice doesn't go into much detail about this use), and used for third-person oblique arguments whose subjects are animate and in the third person:

(2) rígodéhtá
 'S/he is counting them(human).'
 (The e is nasalised - no idea how to type the Americanist symbol, sorry)
(3) yeyíe káidhah
 4.guts
 (Again, the i is nasalised, and the gloss for the second word is also missing in the main text.)
(4) yegts'é rádí
 4.to 3.gives.help
 'S/he helps him/her.'

References:
Rice, K. (1989). A grammar of Slave. Berlin: Mouton de Gruyter.

As a layman in linguistics I found this explanation pretty illuminating:

In English, when we have a non-SAP (speech act participants) involved in the discourse, there is the potential for ambiguity. For example, consider:

“John was in a tizzy last night and got into a fight with Bill. He hit him so hard that he broke his jaw.”

Here, it isn’t clear that who broke whose jaw.*

In languages with what’s called an “obviative” system, however, there is a means of marking two different 3rd persons such that the doer and the doee of an action are clear, even when there are only pronouns in the phrase. These two types are often called 3rd person (or “proximate”) and 4th person (or “obviate”). The details of these systems vary a bit from language to language, but in the broadest strokes, the proximate 3rd person is the topic of the discourse, while the obviative 4th person is used for everything else.