Naphthalene balls are prepared from __________

Eight men L, M, N, O, P, Q, R and S were sitting around a square table, facing the centre. Some of them were sitting at the corners, while some were sitting at the exact centre of the sides. P was sitting fourth to the left of Q. Only O was sitting between M and Q. N was sitting to the immediate right of L. R was sitting to the immediate left of M. L and P were sitting at corners. Who was sitting fourth to the left of R?