1. When you have split personalities (skitzo) and one of the personalities talks about the other personality(ies) as if they are another body(ies).
2. Talking about your alter ego as if they are another person.
Bob has a split personality. His other personality is Tom. Bob said Tom is not here at the moment. So Bob is talking in the 4th person because he is talking in 3rd person but not talking in 3rd person. He is talking about himself but yet at the same time talking about another person. Hence 4th person.