Sentence (1), "I sat besides him," contains an error. The correct preposition is "beside," not "besides." "Beside" is used to indicate being positioned next to someone or something.

Sentences (3) and (4), "I sat at him" and "I sat to him," use prepositions that are not appropriate in this context. They do not convey the intended meaning of sitting next to someone.

Therefore, sentence (2) "I sat beside him" is the correct choice, as it uses the appropriate preposition "beside" to indicate that the speaker sat next to the other person.