In grammar, the fourth person refers to the point of view involving the generic universal pronoun "one", or occasionally "people" or "they" in a generic context ("People say a lot of nothing these days ...").
This can be a little confusing, but the key is understanding that fourth person refers to a more abstract and timeless point of view. This may be complemented by writing in the subjunctive or hypothetical mode:
Suppose one wanted to take over the world. How might one do it? Well, nuclear death rays would be a good jumping-off point ...
Alternatively, you can speak in a mode bereft of time and place:
People these days are getting crazier. One day they'll all snap, and then this world will snap, too.
This is sometimes called the obviative third person. | 
