Based on the previous rounds, the opponent appears to be choosing the move that beats my previous move: in round 1 when I played rock, he played paper; in round 2 against scissors, he played rock; and again in round 3 when I played rock, he responded with paper. This suggests that in round 4, having seen my round 3 rock, he is likely to play paper. Since scissors beat paper, I choose scissors as my move for this round.