1st, 2nd and 3rd person distinctions are not uncommon at all-- found in Spanish, for example, and many other languages. That's where Old(er) English "yon(der)" fits.
A forth distinction doesn't fit very well within the sense of 1st/2nd/3rd distinctions. So it must be something else.
Deictic systems can be organized in many ways, not just based on persons.
My guess is, assuming the persons analysis otherwise fits, it would be, as I suggested above, some unknown third person, an indefinite/distant "yonder". In some languages this might have to do with referent tracking (so-called "4th person") or with evidentiality (for example, is it visible?).
Other languages just do things differently. Some North American languages have around 7 different demonstratives that relate to geographic locations (river, mountain) or relative directions (up-river, down-river, across-river) within that environment.
Short answer: you'd need to actually look at what it means. Otherwise we're just guessing. There are probably infinitely many possibilities, and I don't know of a "standard" 4th demonstrative, nor would that necessarily apply to this language.
